I understand how you feel. To retrain yourself, you're going to need to SLLOOOWWW down. You need to clear your head and then set your mind to one thing: learning to play in a new key. The only thing that's holding you back is your mindset. Your fingers can be trained to do whatever you want, but if your mind says, "This sucks and it's frustrating," then you'll hit the same brick wall you're hitting now.
Seeing that you've been playing in C for 6 years, you shoud know a plethora of chords. So, you need to take those chords and transpose them up a half step to Db. Learn your basic 1-4-5 and 2-5-1 moves and then the rest in Db. Then, do it in D. Then, in Eb, and so on.
If you have a keyboard and some time, then you can do this as long as you clear your mind of everything else and relax on this one thing. As I said, the only thing holding you back is your mindset.