Or maybe her opinion of things carries more weight than your own to you. Perhaps you mean you have trouble identifying and expressing the feelings you have, or you're a perfectionist, or you're afraid of being hurt, or you are loyal to her when she doesn't deserve it. Without that info, I'm just going to discuss your problem as if you were a guy who is longing for a resumption of his relationship with his girlfriend. This relationship may or may not be good for you and your girl, but I have a sense that over-analyzing it and yourself isn't. If you feel you really do have a problem with co-dependency, I'd urge both of you to visit therapists who can both deal with that issue and tell you when your actions are basically a normal situational response.
Many people have a fear of being alone.
I'd venture to say that very few of us, faced with a breakup and being by ourselves, are totally happy and even-keeled. After going out for two years, it's understandable for both of you to be feeling low or crushed after only two months. You've handled it alone and she, perhaps more dependent on others in general, has handled the situation by taking up with a new guy. I think what you need to do is to think about yourself for a while. Don't use being a friend to get sucked back into something you don't feel strong enough to escape.