Need help

Me and my love had a rough patch in are relationship where she left me and got with one of my friends. A year later I finally get her back and I couldn't be happier but the past haunts me. I can't get over it I feel like I can never trust her to stay. I always have negative thoughts about her running off. And I just can't get over the fact that she left me for a friend. Any advice on how to get over this please!?!!? I don't want the past harming my relationship