It's so easy to live in a world of what if's but how about living in the world of 'now' ?
It's such a simple concept yet so hard to grasp and embrace at the same time since the past makes up the now, right?
What I'm saying is, you can't let the past dictate your present or future actions. Especially when it comes to regret, loss, etc. it is a self destructive path that ultimately leads to the same place every time.
You must work on yourself as corny as that sounds. Find what makes you happy, who you love, do what brings out your humanity and maybe try to make this world a little better for others and will give you some purpose? Twas' the case for me at least
If it's stress or work that has you going, don't be afraid to cut ties or call out/maybe even not go back! I've done plenty of things for the sake of mental health and I don't regret a single one. You're more important than stress