Have you ever made someone regret leaving/taking you for granted?

You cannot make someone feel anything including regret. It is best to take actions based on what is best for you. If someone else ends up feeling regret that is on them.

I have had two men come back to me wanting me back after they dumped me thinking the grass was greener. Nothing I did really. Just their own regrets.

Yes, nothing you can do and if someone doesn’t want you, you should not waste one more minute wanting them or wanting anything from them. In doing so you are allowing them to control your emotions and that only harms you. They have moved on, you need to move on too.
Rejection is never personal, people change, relationships change and, especially in peoples younger years interest can be fleeting.

Yes, but it’s a shallow, petty victory. The best revenge is living well.

Right, sorry. Really just wanted to make sure that you guys were aware.