If you often find yourself missing someone who was toxic and didn't treat you right in the end, ask yourself: What is it exactly that you miss?
Because if it's something that they did and over time just stopped doing (for example, they were caring, affectionate, loving, etc.), then you don't really miss that person. What you miss is someone who they pretended to be.
You need to understand that good memories are what trigger these feelings. Memories that were created during times when that person was treating you right, when that person was caring and loving.
And if that person turned out to be someone different in the end, then that person was never genuine, that person wasn't real. He or she was just wearing a mask, and now that mask is off. You need to realize that you're missing someone who doesn't even exist.
If your relationship was toxic, don’t allow its good, but scarce, memories drag you back into it. Never forget the many painful days of that relationship. Never forget that the same misery awaits you if you should slip back into it. And never forget that you deserve better.
Don’t sacrifice your future for some fantasy.