I'm sorry this is bothering you so much. Ruminating about the past regrets is the worst.
This is a basic advice, but I think the best thing you can do is just to focus on doing the best with what you have now. Imagine that you started to play a videogame character and their story begins today, at this point. You cannot change the characters backstory, you need to do the best you can with what is given to you. Or at least do something with it.
There is no way of knowing how life would play out if you made different decisions. Sometimes I'd like to see a simulation of what would happen if I took a totally different turn in life too, but unfortunately there is no technology that let's us do it. There is no guarantee that if you'd make all the seemingly "right" decisions, you would really be better off at this point in your life. For all you know, opportunities you never took might've never work out for you anyway.
We feel regret because our brains are evolved to learn from experience and not shove our hand into the fire again if we already know it burns. The purpose of regret is to make some conclusions. Ask yourself - is there anything you can learn from your regret? If yes, then you should probably do your best to do things differently this time - take opportunities, work hard, pay attention to people around you and look for the ones to connect with, etc. If it is about a particular random chance you didn't take that is not going to present itself again, then it is counterproductive regret, and then see paragraph above - there is no way of knowing that things will really work out better if you took that chance, for all we know it could've been worse.