What am I supposed to do, I cant decide between two girls that I truly care for!?

Somehow it seems like you just answered your own question. In summary, you are still in love with Katie, but you cant take the fact that both of you are constantly at loggerheads, while Nicole is the "Safe" option. It is not fair to Nicole if you continue moving on feeling this way. If Katie still feels the same way about you... then you should truly work things out.