When I was young people had high expectations for me. Supposedly I was gifted. I scored high on every test I was given. All of my friends were the type that ended up going on to Ivy league schools.

 I've made a lot of mistakes. I've gone to two different colleges, got kicked out from both. Those smart friends of mine, well I pushed them away. Why? I can't even remember why.

Now I'm a part-time grocery store cashier. Most of my friends are pretty sketchy. I've been involved in many sketchy things. I've seen some really crazy things.

Its like I've chosen this life for myself, and I don't know why. I try to find explanations in my head, but I still just don't understand what I'm doing.  What is wrong with me? Why can I not understand my own actions? I keep trying to turn my life around, and I fail every time. Now I give up because I feel that my failure is inevitable. 

Sometimes I think it was the pressure to meet those high expectations people had for me. Sometimes I think its because I get bored too easily. Sometimes I blame it on my ADD. Sometimes I blame it on anxiety and depression. For all I know I could just be too immature.

I really can not keep living my life like this. I don't know what I should do. I don't know who I can turn to for advice. I feel really lost and I have for a long time.

Anyone have any advice or words of reassurance? 

It's sounds like there's a part of you that wants the current life that you have. To move on to something better, you'll have to get rid of that, and set your mind on what you want to do....

Exactly what epsilon said. You must set your mind on a goal and push everything else away from you that distracts or is an obstacle in your path. Seems you need some focus in your life. Usually I would tell someone in your situation to pray and talk to God about this, but I really don't want to offend you. I am simply saying what I would suggest.

As a complete loner, all I can really say is you need people for support, and help. What about those people who had high expectations for you? They obviously wanted you to succeed, and they might help you if you ask them. To escape the life you have now, you first need to figure out what kind of life you want to have and have the people who care about you help you reach it. Try going back to college. It doesn't have to be a university, maybe a community college or technical school. From there, proceed and establish as many contacts with people as you can. You need them to climb up the career ladder. Also, comfortability is a dangerous thing in this case. People get stuck with their life because they become comfortable with it, even if it is a life they may not have wanted.

Sounds like the story of my life actually.

I know I could have done so much better for myself, but that's the thing really. You need to stop beating yourself up for what could have been and try to make the best of what you've got.
Just find someone/something that makes you happy. Figure out a realistic goal and go for it.

Second school I went to and got kicked out of was a community college. Those who had high expectations have already given up on me. I end up frustrating anyone who tries to help me.

I started acting the way I do at a very specific point in my life. I was on an ADD medication that made me sick, I pushed away my friends and became a loner, my parents hit me, I got extremely depressed and suicidal. I ended up getting diagnosed with post traumatic stress and refused to take anti depressants and pretended to be better to get out of counseling. I lost all interest in all of the things I used to do. I don't really understand why I can't make myself do the things I used enjoy anymore. This was about 10 years ago.

I don't really see how that can explain my perpetual inability to succeed though. Or my being drawn to a sketchy life. But hey, its one of those factors I sometimes blame for this. I don't think I ever fully recovered from that because I was a stupid kid who wouldn't cooperate with those who were trying to help.

Sounds like you went through "the shit" really young and you might not ever fully "recover." I put the word recover in quotes because it's bullshit. Much of the discussion on these issues ends up being too demeaning to be helpful. It doesn't mean you're worse now. It just means you're different. It might even have made you a better person in ways you don't realize or understand yet. Seeing as how I'm a stranger on the internet, all I can really say is I hope you don't worry too hard and get into the downward spiral of anxiety/stress/depression that can last forever.

Also, fuck success. Successful people aren't happier than you are. You can make yourself happier than any of those nutjobs. Best of luck to you.

Maybe you have chosen the wrong educatiion path as it sounds as if you arent interested in it at all, you could try studying something which is more linked to your hobbies or interests.

Every week I get people telling me that if I'd tried at school I could have done anything. But I like my life as it is, I have a girlfriend of 11 years and a 4 year old son. Maybe I could have been a successful doctor or a lawyer or whatever, but then I wouldn't have what I have now, and no money in the world could replace that.
Do what feels right for you, and fuck other people expectations.