Regret is the ever occuring phenomenon in BTC world. It may sound cliche, but this attitude works for me everytime: I grieve, I extract a lesson (or accept what's irreversible), I move on. Easier said than done though. If you still have BTC, I wouldn't dwell on this too much anyway.
If you will regret every time you sell your btc, then, that's really not good. Getting crazy on what should have done. For me, I learned my lesson, once I sold, I don't go back to that time that I sold it. I just moved on and look for other opportunities. It will just give you headache if you will revisit your trading activities.