How do you frogot about what we used to have

I've been with my boyfriend for 3 years we were engaged at one point we used to talk 247 he wouldn't dream of cheating or talking to other girls and now he ignores me all the time wont talk to me vheats every single chance he gets and I cant helo but think if i stay or what can i do will it go back to how it used to be what am I doing wrong what caused hin to be like this..how do you get over thinking this way and just leave the relationship