Why do my negative thoughts out weigh the good?

I am in this situation too, i try to be positive but like you i worry and negative thoughts appear. It is true that that if you pretend to be positive/ confident you start to feel positive/confident, because once before an oral presentation i was freaking out as usual, but i walked up confidently and actually started to feel a bit confident. i have tried saying positive things to myself, but i dont stick with it so have never been successful. w should both make a goal that for at least two weeks we switch every negative thought with a positive one and see if it works!