first vs second love

You answered your own question. Why would you want to go back with a toxic boyfriend? If it was meant to be, it would have stuck. Be grateful you got away from a toxic person and found someone who loves you without trying to control you. This is a true blessing. I think you would be very sorry if you went back to that. And it makes me wonder if you love your boyfriend now because if you did and were satisfied with that relationship, why would this other guy be in your mind? Perhaps it's only that you're feeling guilty that you left him or vice versa I don't know, but it would be a big mistake to go back to a toxic relationship.

Acknowledge the things that were good in your first relationship.

Acknowledge the things that were bad.

Don't forget them, but relegate them to the past, where they belong.

Then live in the present.

This is common. Everyone thinks about their former loves and what-ifs and the like, no matter how happy they are with their current situation.

When you love someone, that person carves out a them-shaped hole in your heart, and that hole never goes away. It's just that, if you're lucky, someone else will carve out an even bigger them-shaped hole. That doesn't keep you from feeling the emptiness in that first hole sometimes.

Amnesia regarding ex's is common. We tend to forget the REALLY bad things, and our own misery and ONLY remember the good things.