Ok so me and my girlfriend have been dating for almost 2 years. When we first dated she was sweet, kind, caring, understanding, everything a guy could ask for, but now, she just can't admit when she's wrong, she always has some stupid excuse for not doing any work in our relationship even though she agreed that it would be 50 50, sexually she is always promising to do something to me but ends up never doing anything at all for some convenient reason. And when a perfect moment comes up she just puts it off for another day and still never does anything. When I talk to her about it, it seems that everything I say to try to help her just goes right threw her head and she just tells me what I want to here. She just doesn't want to admit that she has a problem and that she has done nothing to fix it. I've tried to ignore it but then it got worse. I tried to stop talking to her and stop doing what she wanted and she just started acting worse,she talks to me as if she doesn't care about me at all but she is so convinced that she cares more about me then anything else but from what I can see, I just can't believe her. Please someone help, I can't break up with her again, I just can't live my life without her. I just want her back, not this new person she has become, I feel like its my fault. How do I fix this?

It sounds as though she isn't as vested in this relationship as you are. There is nothing you can do to fix it.

Communication is key. If the two of you have communicated about this, and she is unwilling to change, there is little else you can do except to go your own way without her.

Everyone starts kind and caring - that's romance. Love takes maturity, understanding, compromise, and mutual respect.
We can't tell if she is the problem or you both are. She isn't here to tell her side. I always cringe when someone says 'she just can't admit when she's wrong.' Maybe you can admit when you are wrong, maybe you can't. You haven't really said anything specific that she does, except to put off sex.
You can't change people. You communicate the best you can to work out a problem, and if it doesn't work, you ask yourself if it's time to leave or if you can accept her the way she is. You can't go back in time to a memory.

"i can't break up with her again, i just can't live my life without her."

You can and you can. Waiting for her to return to who she used to be or what you remember her being isn't a solution. If her behaviors are something you can accept, stay with her. If not, move on. Life is too short for so much drama.

Some people try telling me I can't admit when I am wrong. I ask them why does this or that have to be somebody's fault where a wrong has to be admitted? Like we have plans to do something but everything goes wrong and before you know its too late to do what you had planned. Like joypulv said we can't tell if she is the problem or if you are or something else, but I will try to give her perspective the best I can.
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You say she is full of excuses, buy telling her her reasons are excuses she feels demeaned rather than validated. Try being more understanding of her and instead of making her feel put down start doing something more like a constructive criticism like if she is not cooking dinner say something like maybe WE could cook a big pot of chili or something on Sunday so that there are leftovers on weekdays that you do not have time to cook.
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My boyfriend was very demanding the first year we were together. I sort of got him away from that because to me love is not what you do for a person and if the person isn't happy with you because of what you are not doing then maybe it is time to look else where like J-9 said. Love is unconditional. I can understand your frustration if she is wasting all her time watching TV, playing video games, hanging out with friends, etc... otherwise maybe you need to start giving her credit for what she is doing and constructive ideas on how to plan her day better rather than coming off as naggy to her. Like Odinn said is her behavior something you can except?