That's a pretty specific situation you're dealing with so I don't know if I have any useful advice to offer. I can just say that throughout most of my life I've dealt with regret the same way, no matter the circumstances. I remind myself that it's a stupid and useless emotion to indulge in, since there's no way to go back and change anything. I tell myself "acknowledge things you regret long enough to learn whatever lesson was to be learned, and then forget about it."

Of course that's sometimes easier said than done, but replaying that thought in my head usually helps.