I have this and all sorts of (little) OCD problems (have I locked the bike up/garage/house/etc).
One tip is to do something unique that you will perform just after closing your bag (As suggested by jonba above). I’ll elaborate: Close your bag, then put your elbow on your knee and say the first two words that come into your head, “OBVIOUS PLINTH!” – for example.
Then when you’re riding you’ll be like “ah yeah I deffo closed my bag because of the elbow-knee obvious plinth thing”.
Next time you do it you have to generate some new actions and words.
Good luck fellow mentalist 😀