Should stay with him or move on??

well we have been together for three yrs and this has been the best and worst relationship i have ever been in. The problem is that he has cheated on me before several times and of course i for gave him.. he asked me to marry him i said..yes..of course..but now he is out of town for a job..and he is always on my mind..im always wondering what is he doing and who he is with..this is not healthy..i know..but when i check his vm..and yes i check his voicemail..there is always a NEW girl on there..i dnk think he is hooking up with them like like but the idea is still in the back of my mind..what should i do...