Why does my ex still affect me?

I am still emotionally hurt from my ex even though the relationship was 3/4 years ago.

The relationship ended badly where they ghosted me in the end. Then 2 years later I saw them on the bus and they came up to me and we awkwardly chatted. A few weeks later we met up at a coffee shop and chatted again to clear the air and to catch up on our lives. It was like he was intrigued to find out how I was doing yet he was in a new relationship. I was single at the time. We didnt keep in contact after that day due to him having a girlfriend.

Then I got into a new relationship and have been with them for 2 years now. I really love them. But I feel guilty becasue I keep having thoughts and memories of my ex.

Recently I received a facebook message from the ex a few weeks back but it didnt make sense becasue I think they must have been drunk. He is still in the same relationship. I stupidly replied becasue i repeatedly wondered why he contacted me but he never replied and left the message on read.

Is it normal to have thoughts/memories of your ex after all this time? Why do you think they contacted me after all this time when they are still in a relationship. It is like I can not let go completely becasue they keep appearing in my life somehow. I feel pathetic.