IF YOU believe in God and always feel His loving and guiding presence, then there’s nothing in the past that happened that you should regret. It is so because God is always in control and good or bad, He knows well and will decide according to His volition.
As is always said, all in God’s time! Believe it or not, He knows best to steer your life according to His decision and there’s nothing that you can do which will deviate the outcome of your actuations from what God has deemed necessary and worthy.
I used to regret the past and one major regret of mine was getting involved with and marrying my late husband. It was actually Waleed who has taught me not to regret anything that has happened in the past. God has always a reason and He always knows best. Waleed said that if I didn’t marry my ex then we won’t be meeting in Kuwait. It was my ex who brought me to Kuwait which led to me meeting Waleed and marrying him in the end.
If the thing you did in the past is very bad you should instead repent but not regret. Repenting will be a good move according to God and one should always make it a point not to repeat the same bad actions you are repenting about. And you should always ask for forgiveness if at all possible and promise not to do the bad move again.