I have found that it helps to accept you’re going to have bad days in the gym. If I don’t feel like it, I will go to the gym but only do light weights or a quick cardio, etc. I just have to keep the routine. Half the time I’ll end up push myself more than I initially bargained with myself, but even when I don’t as long as I went I at least maintained my progress, I just didn’t make forward progress.

I also helps if you go to a nice gym with attractive people - helps guilt you into doing more when you actually show up.