mixed emotions .

I have been hurt a lot in the past. I recently starting dating this guy that i have been friends with forever, and i know we are both very happy with each other, but just like every other relationship there is something in my head saying " oh , are you sure he's not cheating like the rest" or something horrible like that and i dont know what to do because i dont want this to ruin our relationship and its not fair to him. I know deep down he isnt doing anything wrong i have just been hurt so much that im used to it so its hard to beileve that someone isnt actually hurting me. what do i do , i just want us to last and i just want to stop with this nonsense.