Why can't I stop thinking about the person when I know I don't even like them anymore?

Because, you subconsciously do like them. You are just trying to deny that feeling. That is called trying to suppress the memory. You might still like that person but you consciously know you shouldn't, so you're doing what your brain tells you to.

Because of the effect that they have on you. Some people whether they are around or not, whether you like them or not, tend to strike your deepest fears, till your fear persists, the person persists in your thoughts.

Because the good moments that you live with that person are the reason why your brain is repeating the memories again. You want to feel like you felt at that time, but not with the same person.

Because you like the attention the person gave you or maybe you want people to want you. I was like this when I was younger and I dated a guy who gave me so much attention and wanted to be around me. I soon broke up with him (because I found out I never liked him but the attention he gave me and I got bored with his attention) and found myself daydreaming of him wanting me back even though I was relieved he didn't want me in reality but also a bit disappointed. I wanted guys to turn their heads and look at me as if I was the most gorgeous person alive and to want me. A very shallow desire from a person who was insecure in themselves at that time. Luckily I only dated one guy during that time because once I found out who I was, i stopped myself from ever dating someone again even. I don't regret doing that to a person but I know it's not right but however, I do regret that time I wasted being with him. Not saying this is you but could be a thought.

The person used to be a really big part of your life, they were a part of you. They were like a habit and habits are not easy to break

You might not like them, however having strong feelings for someone doesn't change overnight. Even if said person has wronged you in a way that can never be forgiven. The feelings are real or were real and it isn't about still liking them, its about having had an emotional connection with them at one point. It doesn't just go away.

to some extent you have developed an autonomous chain reactuo of train of thoughts about that person. Its difficult and makes you feel irate and feeda anger. I know how you feel. It hinders in your everyday routine and distracts you living a normal life

Sometimes our feelings for people linger, it's normal. Things will get better with time. Continue focusing on things that matter the most and eventually you will not think about them as much.

Maybe you do like them but are forcing yourself not too if you think you do not like someone you can never be to sure you might think you don't think them but you probably still do.

Your mind gets thrown into a sort of loop when you lose a person. Even though you don't like them anymore, you've spent a considerable amount of time with them and thinking about them was your habit. Breaking habits is hard and can be a long process. It needs time, you will think less about them every single day until a day comes when you'll realise you haven't thought about that person for a day or more already. Just know it does pass.