Cheating

My ex boyfriend cheated on me throughout our three year relationship. When I found out he begged me to stay. I did and he continued cheating so I left. I am now engaged to a great guy, but I have trouble with trust issues now. I know he is loyal to me and wouldn't cheat but my mind doesn't think rationally sometimes and I get paranoid. Has anyone else experienced this? Do you have any tips on how to get past these issues?