Why do I always do this☹️

I always ruin anything that brings me happiness, it always happens eventually. I have very bad anxiety, which I had gotten control of but is getting worse lately. I have been with my boyfriend for over five months now and it's been perfect but recently I've been telling myself that something is going to go wrong, that he doesn't really love me and is going to leave me even though he talks to me about marriage and children and being together always and last night I just felt so low, he got upset because he thought he'd done something wrong and kept asking what was up with me but I don't know how to explain it so I just said I was okay and he knew I was lying. Now he's really hurt and I don't know what to do I can't lose him just because this anxiety is controlling me.