Well, we sometimes say that we don't feel regret. But deep inside, it's not very uncommon to feel the regret. As society sees regret as a sin. We try to hide our regrets, we don't approve of it. And later we find ourselves in a conflicting condition with our mind when we become confused about whether it's okay to have those regrets or not.
If you ask me, I will say that I believe having regret is totally okay. And we should actually have some regret. Regret means we are living, doing mistakes and we are not stopping. But the most important thing is how we deal with regrets?
Accepting regrets is the biggest thing. Not accepting regret can lead us to further mental issues and sufferings. So first accept it, face it and approve it that you wish you could have/haven't done this.
Once you accept it, then try to take action to move on. Accepting doesn't mean you have to try hard to achieve that or fix that what regret you felt. Rather, fix up with your mind and move on with life.
It's okay to have a suffering past or not achieving what we really could. It's okay to have some thought that 'what if'. But move on. It's your life and you can decide your present. Act on your present so that the same thing doesn't happen again.
Sometimes, it depends on time what we can't fix anymore. I think that's also okay. You don't have to achieve everything. Learn to be happy with what you have. Accept your regrets, don't hide it, deal with it and move on.