Showing up to train on days you don't feel like it is what separates consistent trainers from inconsistent ones. The best training year contains many sessions where motivation was absent. Discipline beats motivation.
How to make it easier
Same time daily (reduces decision). Pre-set workout (don't decide what to do that day). Specific minimum (15 minutes counts; often expands once started). Public commitment (training partner, group class).
The session you do unmotivated counts the same as the motivated one. The body doesn't know the difference.