It's up to you, as to whether or not you believe my sentiments, but I really do understand where you're coming from. If what I say seems harsh, I don't intend for it to. I'm trying to show you what there could be in your life, if you're willing to open up your ears, and listen for a minute.

I couldn't ever understand the loss of a parent, but I have lost many pets in my time, and many more friends.
There are no two situation alike in the world, so no one can ever understand exactly how you feel in this place and time, because they were never in that place and time, with those people, doing those things. But you have to understand that when people tell you they feel where you are coming from, they're as unsure of what to say as you are. You say you don't know what to say to tell them how you feel, because you are afraid of them not believing you. Then you say that they won't believe you, because you were never up front with them before.
If you want them to believe you now, you have to make them see who you are. I truly hope you believe me when I say I KNOW how hard it is to open yourself up to the world, and expose your deepest feelings, your darkest thoughts, and your scariest memories, when you're utterly terrified of being rejected MORE. I've been teased my entire life too, from the time I was 8, until now. I'm STILL teased, and called names, but they've stopped hurting. I've had to deal with going to school with vinegar in my hair, and oil, and chemicals, because my mother couldn't get rid of a lice infestation. I walked through those jokes. Several years later, the infestation was back, after spending a year living in a homeless shelter with my mom and sister, and after months of treatment to no success, I had to cope with having every single one of my 14 inches of hair shaved off, and hear the laughs and scoffs behind me as I went into the girls bathroom. "Omg, that's a GIRL?!" "Lookit that dyke, bet she's gonna rape some kids in there."
So please, believe me, I understand the ridicule, especially for my weight, too. But the one thing you should know, if you have passed them off as nonsense, is that old sayings may sound stupid when you're young, and cynical... But they hold so much more truth than they are given credit for.
"Sticks and stones may break my bones, but words will never hurt me" is the best one for this paragraph. Sticks, and Stones, and Knives, and Guns kill far more people than a few personal jokes, or pointed fingers. If you let them know they're hurting you, it will hurt you more, because you will live in fear of those jokes. Instead of running away from them, STOP. Turn around. Face them. Don't threaten the jokers, don't throw insults back. All this amounts to is more pain, that could end up being physical. Instead, throw their OWN jokes right back at them. If they call you fat, Grab your stomach, or thighs, or arm and say something stupid. Like a quote from Santa Claus, and say "A little fat? Does this LOOK like a little fat to you??"
They'll laugh with you, and you will get carried away with the laughter and end up able to laugh at yourself, which is one of the most beautiful things in the world.

As far as your psychological problems are concerned, remember that they are just that. Psych. The best old saying for this, is "Mind over Matter".
If you want to think scientifically, your brain controls EVERY part of your body. Every feeling, emotion, and movement is nothing more than an electrical impulse, charged through the body by the brain. So, if you have control of your brain, you will have control over your feelings, your emotions. When you're feeling most depressed, stop, and think logically.
Step outside of your mindset, and picture what you would look like, if you were a stranger sitting next to you.
Nervous Wreck?
Depressed?
Angry?
Dangerous?
You have to think about how the world would look at you, before you think about how the world will see you. They have to take a good long look before they see ANYthing. Make sure you can understand where they are standing, and you'll come a long way in understanding why they're at a loss for words.

"I understand" is nothing more than saying they understand the meaning you are trying to convey. It can be an empty sentiment, or it can be an emotional bond. It's up to you whether or not you accept that extention as a hand up, or a shove away.
"I know" is more what someone would say if they truly knew how you felt. If someone says that, ask them to elaborate, if they are able. Sometimes the strongest bonds are made from grief. Two or more people that look each other in the eyes, and connect on the grounds of common loss have nothing more to lose. With that, they become rungs on the ladders to each other's healing process.

So, while no one will know EXACTLY how it feels to lose their father at 6, and have promises broken at such a young age, and to be bullied... neither will anyone have lived life EXACTLY as you did.
No one will be able to help you until you can see that.

I've already written more than probably any of you are going to read, but at you specifically at least Moon Dragon, I want to extend a hand, and welcome you to my world. I want to show you what I have healed from, and show you how I have healed. I want to let you explore the land I keep my heart, soul, and feelings locked up in, and show you the safeguards I have in place.
But I can't do anything until you are willing to step out of your own life, and open your eyes, and heart, and MIND to all the other options there are.

I'm not saying I'm a ball of sunshine, and daisies, or that I poop rainbows in my free time. Not by far. I have my moments that I look around and see all the different ways I could kill myself, and make it look like an accident. All the thoughts mulling in my head, like "Who would honestly miss me?" and "Who would visit me if I were sick, and dying, in the hospital?"
But I AM saying that I can help you put all those aside for the rainy days, and build something out of them.

If you think no one will care, think about WHY they wouldn't care.
Set aside your ego and pride, and think logically.... What have YOU done, that they would feel lost if they lost you?
If you don't think of anything, maybe it's about time you started GIVING everyone a reason to miss you. The more you do it for them, the more they'd miss you in their lives, the more complete you will feel, and the happier the days will look.

My friends and I started a GSA Club in a very homophobic high school, to represent the Minority. Day of Silence, RAINN, etc.
In this GSA, we thought of ways to memorialize our gay friend who had killed himself just DAYS before he turned 16. Why? Of course we don't know. All we know is he was bullied and mistreated, in home and at school, since he was born; and that the night he died he fought with his dad about DYING HIS HAIR; and that the entire school was quieter without him. We decided on a memorial garden AT school, to pay respects to in our free time, and motivate us, and be a tool to teach in biology classes, and give us something to do with our time. Hundreds of volunteers showed up every day during the summer, every saturday during school time, and worked the fingers to the bone, in hot summer months even, to come together and make a vision a reality. People came from all around the county, the state, and even a California news crew came out to film us, and interview myself, and another girl who had worked from day one with me, and a few others, for the garden. A boy in GSA asked his mother to help us, and she agreed. A few weeks later he was killed from injuries sustained from a car accident on the way to school... on the Day of Silence. The girl the California film crew interviewed with me died from a car crash the next year. But we all stand proud together, and continue to work at the garden, and share lunch together every saturday we are able, to pay respects to each other, and our mutual losses.

So, I DO know what it feels like to hurt like you do.
But the best advice I can give you is open yourself up.
Tell everyone EXACTLY how you feel, and make sure your feelings are known, and not walked over.
Be straight, upfront, and demand respect for the things you do.
The pushover is never remembered, but the leader is, so just stand up for what you believe, and do what you know makes YOU happy.
And, most importantly, that grief will NEVER go away, as I'm sure you're well aware of. But what you DO with that grief can make things better, or make things worse. BUILD on that grief. Help others, show you care, turn things around. It's all up to you, and the ball is in YOUR court.