Just tell yourself you’re not going to lift heavy/are going to lift light. When you don’t have the motivation (or whatever) to go to the gym, tell yourself this. Like, “I’m not going to work too hard. It’ll be easy.” Works a treat. Then even if you end up doing that (which is unlikely - I always end up getting in the mood hehe) then at least you did something and kept the momentum.

Edit to say: The hardest lift is getting off the couch.