How to stop being insecure and overthink?

I recently almost lost a girl I love because I would start argument for no or small reason. I would get jealous fast.and become bitter. Last time i got angry I hurt her. I couldnt sleep that night,because of what i said. And I tought i dont deserve her.

Im really ready to change. I made a huge apology to her. She needed 3 days to start ti liek me again. And on this third day I got a bit jealous and angry again,but i didnt show it. After a few moments I understand than it was either not a big deal or anything at all. But im scared I might lose my temper again..what to do? How to start? I want to change. Im ussualy a confident guy its just that i care so much for her...