Anxiety over fiancé :(

I should be the most happiest Ive ever been, I proposed to my fiancé, she said yes, I love her more than anything in the entire world, but all my brain will do is tell me that shes going to cheat on me, shes going to meet someone else. She has given me zero reason to think this, but I cant get it out of my head. I think a lot of it is, I am very suicidal, and I think if this happens Ill be annoyed I stayed alive, because it was just going to lead to more hurt. Im relying on Valium and alcohol every day to just stay afloat, I feel lost, I feel awful