There are (says Moom) different types of regrets. On the one hand you have regrets for what you sacrificed when you conciously chose to do something else. On the other you also have regrets where you didn’t sacrifice anything for something else, you just did nothing and missed an opportunity.
Moom says (paraphrased in my words) that you need to minimize your fretting in the first case and just get off you butt in the second. I can agree with that.
This is all connected to the thing with Teshuva. Teshuva is about reviewing your past and resolving to improve for the future. It is about a kind of spiral where on every orbit you are getting closer to what you want to be rather than just aimlessly circling your target.
If you made a wrong choice then don’t fret, fix it or do it better next time… and if your problem is making your decision for action then analyse your problem, resolve to act and get it done.