Confused with my bf and my ex??? help !!!?

so i was off and on this guy for 4 yrs he cheated on me ... hit me and always called me dumb and as if he knew everything... and i finally had the courage to leave him and i am now dating this other guy who does everything for me but is fat and he is really dumb i mean he needs to p this state level test called TAKS and he failed it 5 times but now my ex contacted me and he is telling me he is a changed person and to give him another chance and he will prove me wrong idk whty to do help me what should i do ???!!!