Is it possible to stop being insecure?

I am 25 been with my boyfriend 8 years and things are crazy i am very insecure and he has cheated in the past and now he seems to be changing he is 34 this year and he recently got out of jail i helped him to get his job back and 3 months later he is made shift leader at his job(Zaxbys) possibly manager i am insecure that he may leave because i accuse him of cheating he claims he has not cheated at all and that he is trying to change and marry me but he claims my attitude is bad and i accuse him of too much to get married right now,my insecurities are from him cheating he used to sell drugs and the women he slept with are nasty,i can say that he has been home everyday soon as he gets off work and if he does go anywhere its not much and he comes home before 9 pm he is here on off days also but im stll insecure that he is cheating i dont know w hy i cant stop its tearing us apart but i cant help that i feel this way how can i become more better and not be insecure what can i do to make our relationship better to prevent a bad break up,we have two daughters one he has raised me since she was six months and his biological daughter who is two. Can people change?I really wake up feeling miserable and i do argue and accuse him of things almost daily i really almost hate myself i need help with this insecurity living inside of me i am a beautiful woman i dont understand ,i have stopped before for a while but the more i think of the past i bring it up