i’m in a relationship but still miss my ex

I was in a relationship for about 4 years. He broke up with me in february and I suffered a lot and he cut contact w me completely. I got into a new relationship in may and although i still missed him quite a bit I really liked this new person and he makes me really happy. Really really happy. but sometimes i can’t help but still miss my ex at times which is normal i think. last month my ex messaged me saying he heard i had a new bf and that he was happy for me. he said he missed me and although we could not be together anymore he would like to maybe be friends one day if i ever needed him. it made me really sad and it confused me a lot in my current relationship. i decided to get over it and continue with my boyfriend now but recently had a conversation w a mutual friend of me and my ex and she told me that he still asks about me and says he misses me. I don’t know if me staying with my current boyfriend is unfair to him because of how that made me feel. Even though I could possibly never get back with my ex because of the way things ended between us I can’t help but feel guilty with my current boyfriend.