Trust issues

I have serious trust issues , and I've been with my boyfriend for 9 months . He always tells me that he's not cheating . I know the password to his phone , I check his Facebook and everything else but I don't find anything . But of course I always have to be the negative Nancy and think oh maybe he's deleting shit lol . I honestly have to get over this trusting thing and I honestly don't know how . I always think he's going to find prettier and skinned girls . But he tells me that he loves me and I'm beautiful and he wants to spend the rest of his life with me . I'm just scared that what if one day I actually find out that he's cheating / cheated . I'd literally die from a heart break !!!