Some reasons not to have regrets.
If that hadn't happened exactly like it did, I wouldn't be here now.
Worrying about this is making me enjoy this current moment less. If I keep this up, I'm gonna regret it later..
Dwelling on this is not a good use of time or energy.
Did you learn something from it? Yes? Good. No? That is ok, too.
Stop ruminating, fixating, being stuck on some past thing. Even if it had gone different, it very likely would have ended up worse.
It would certainly be worse for you, dear reader, because then we wouldn't having this conversation.
Also see: regrets counter.