Why does it feel so hopeless?

I just feel so trapped. It feels like I’ll never be able to properly enjoy spending time with my boyfriend ever again because I feel too burdened by the obsession that I’ve cheated on him. It’s really starting to take a toll on my quality of life and I just want it to stop. I can’t have any moments where I can feel at peace with the man I love. I can’t just lay in his arms and feel that everything’s fine and there’s nothing I need to worry about. I’m so tired. It just feels like it won’t stop until either he or I leave one another, which is the thing I fear happening the most. I’m so scared that, if I spend my life with him, these barely ending thoughts will never stop and I’ll still be feeling like a disgusting cheater and fixating on particular things at like 50. All i can feel is guilt, shame, fear, and doom. I feel so doomed.