@3or4monsters has a very good idea, I think. I look forward to hearing more about it. You might try looking into Acceptance and Commitment Therapy. The Happiness Trap is a good place to start. They suggest ways to deal with “shoulds and oughts.” I haven’t gotten that far in the book, though.
I suspect is has to do with separating yourself from your self-judging thoughts. “I’m thinking that I should run? Isn’t that interesting?” I.e., your essential self is separate from the thoughts about what you should do. You can choose to pay attention to those thoughts or not, depending on whether it is useful. Useful means that it makes you feel better. In this case, thinking you should have gone running is a way of beating yourself up, so it ain’t good. On the other hand, as @3or4monsters says, thinking about how good that run felt—that feels good.
Praise or punishment are kind of both unhelpful. You probably don’t want to run because you get “props,” nor do you want to run because you’ll beat yourself up if you don’t. You want to run because you want to run. You enjoy it. It makes you feel good. You look forward to it. That ache in your legs when you do run feels better than that ache that happens when you don’t run.
After a while, you’ll start to get used to not beating yourself up, and you may notice that it is easier to feel better about yourself. I really want to lose weight, so maybe I can take some of my own advice. I did ride my bike to work today, which I really enjoy, and haven’t been able to do in over a week due to this incessant rain.
Another trick that works for me is just not thinking about the judgmental thoughts. When they appear, I don’t pay much attention to them. I may ignore them. But usually I tell myself that it’s not worth thinking about. When I don’t judge myself, I can just be. Not good. Not bad. Just daloon. It keeps me more on an even keel when I do this. I still get to those spiraling down thoughts when I do think them, but I see that, and I stop doing whatever it is I’m doing that brings it up fast enough to keep the bad stuff away.
Well, I could go on, because I’ve just started thinking about how this might work, but for now, I’ll leave it alone. If something urgent comes up, I’ll let you know. @3or4monsters—please do say more.