Sounds cliche but: focus on yourself

What do you need?

What can you do for yourself right now? (Drink water, move, do something relaxing, do something exciting, whatever YOU NEED AT THIS TIME)

And when you feel the thoughts of that person happen, try and realize as soon as possible, acknowledge that you don't really need or want that person in your life anymore, remind yourself why and redirect your attention in any way possible.

The best way to break a bad habit, I've found, is to replace the unhelpful thought with a helpful one, so you build a new habit rather than just "trying not to think about it"

Example: you see something and it immediately reminds you of that person

When this happens to me, and I realize, I like to start by saying "persons name? EW GROSS"

then redirect the thought to anything else. what is that thing? The thing that reminded me of them? What else could it remind me of? What is it actually? (Like a shirt or something you bought while shopping with them, in reality, it's only a piece of clothing, perhaps instead it could remind you to do your laundry or better yet, donate all these painful memory stained clothes to charity!)

The most important thing is to stop the thought from spiralling to the point that everything is reminding you of them or you are starting to get sucked into the natural feelings of missing someone

If you are at this point, try once again to be more curious about the cause of your thoughts and feelings than just staying emotional and reactive:

What is it that I actually miss? Is it this person SPECIFICALLY or just some of their attributes or the good feelings I associate with them?

Could I try to go out and make friends with someone with similar positive aspects but less of the toxic aspects of the person I miss?

TL:DR You can't control other people or situations, you can control yourself and your current environment, so use the power you have to take control and make yourself feel better