No interest in sex sometimes

I’ve been having some issues in the past few months with getting in the mood. My fiancée is amazing and I’m very attracted to her, always have been. But lately, whenever we get into bed, sex is the last thing on my mind, and sometimes I just hope she’s tired and not interested either. If she really wants to, I’m able to perform at least some of the time, but I have had trouble with performance before, I think that’s why I have no interest now, I just feel like I won’t be able to get her off so what’s the point.

It’s absolutely no fault of hers, I just wish for her benefit I was a little more up for it sometimes. Any tips for getting in the mood late at night?

Based on what you wrote, it sounds like what’s getting in the way of enjoying sex is meeting her expectations. If you haven’t already, it seems a conversation with her about her expectations is overdue… and if you did already talk about expectations, revisit it.
A psychological phenomenon I learned about, years ago, is that when we, as humans, come to believe that we are not going to be successful, we tend to assume we are inadequate and we give up. The situation is uncomfortable and we would rather just avoid it.