Mood swings. Present! i just need help!

Hi I am having huge mood swings! I try to concentrate on work, still I keep on overthinking about some things that are of no use in present. I keep thinking about my GFs past over and over again, how badly she treated me which is useless since she is all grown and changed and loyal with me. Additionally if somethings doesn't happen according to me, I start think how I am doing everything to keep everyone happy and how everyone else is just using me and not thinking about me! This is affecting my relationship! Please help me tell me how should I just stop overthinking! I am very stubborn person and when something does not happen according to my thinking, I go ballastic mentally and I just stop talking to people and start overthinking all small things! I live in past so much and it I hurting my present! I just need help!