How should I deal with this tangled situation?

I have a group of friends i've been hanging around with for most of my high school years, right now i'll be heading into my senior year. To start off bluntly, they treat me like trash. I'm constantly made fun of and called names, regardless of me asking them to stop. I'm a sensitive kid, I take a lot of stuff personally that other people don't, but I think being relentlessly put down will make any kid lose their self esteem. Now, i've done a lot for these guys. I was the support system for one of them when his girlfriend and him were having problems, I went to great lengths to keep them together and quell their arguments. I also got this friend a job when his other one's business failed, I was the first one to refer him to my boss. He was hired the next day, I got him back on his feet. Another friend i've covered him on his shift at work at least 15-20 times when he's had things to take care of. He always tell me, "I owe you big time." I've also been his shoulder to cry one and talk to when he's had relationship problems. My third friend, I drove him around from place to place for year and half before he got his license, free of charge on gas. The last friend, I made the world hop basically for him. I let him live with me for a year and a half, I gave him food to eat, water to drink, and a bed to sleep on. He has major financial problems, he was abused by his mom and his dad has the most horrible credit ever and can't hold a house for more than a year. When he was homeless I took him in and gave him $300.00 when his dad found a house to rent for food and such to get them started. Now, a few years later i'm planning on studying quantum mechanics and theology for a career. So I do a lot of readying on that subject. I've also got a girlfriend I like to spend a lot of time with, the situation came to a climax last week when I found out they lied to me and used me to work their shift for them so they could go out on the boat. I wanted to go with them, but they told me they wern't allowed to go now and asked if I could work for them. I said sure, I don't mind, I found out they went out on the boat. I was kind of upset, so I confronted them and asked them why they lied and used me. I was laughed at, it sorta pissed me off. After school one of them pulled me aside and said the reason why was because "i've become full of myself" and "I always talk about stuff no one cares about." I was surprised, I asked for examples and they couldn't give me any. Maybe this is a personality flaw of mine that I hadn't noticed, and I kind of see that some of the philosophy crap I bring up in conversation can zap the fun out of it. It took them a year to tell me this though, while they used me to drive them around, give them money with no hope of being paid back, and I gave one of them a place to live and quite a bit of money, while always being tormented and laughed at when in a group. Constantly being made fun of has also made me quiet, i'm afraid to speak for fear of whatever I said being turned against me in a joke. They want me to be less uptight and more funny, more like I used to be in years past with them. I did grow older now, i'm 18 and i'm starting to look towards life. I'm less immature now, I have responsibilities to tend to. I can see my personality flaw though, they could be right that i'm like that. I feel like they're setting expectations for me to live by instead of me setting expectations for myself. They want me to be a certain way and won't accept me for who I am. Who really wants to hang around with a kid who's full of himself though? I will work on that. I've also become co-dependent too, without them I'm back to square one with no friends. I'm looking for advice on how to handle this, I feel like to I need to put a mask on and act differently now. Sort of change my personality to fit their needs so I can keep them as friends. Any answers would be deeply appreciated, i'm very confused.