Unresolved feelings for my ex-girlfriend

I(24M) am in a relationship with a girl(26F) from my class in my university. It's been only a few months. It was really difficult for us to get together in the beginning because she had a boyfriend at the time we met. She cheated on him with me. She wasn't happy in that relationship. After that, they broke up and now I'm with her.

I am happy with her. She's very nice, happy, talkative, cheerful and extroverted. She's great in bed. I am very comfortable around her and vice versa. We enjoy spending time together.

The problem is, I'm not madly in love with her. I don't want to spend all my day with her. I do want my own space at times. I realise it's the healthier thing to do, so I guess that's not a problem.

The problem is, my ex-girlfriend, who was my closest friend for a year and whom I was madly in love with, wanted to spend the rest of my life with, thought was the One, broke up with me two years ago during the lockdown. I cried over her and was hung up on her for two years. After lockdown ended, and I met more people in my new university, I found my current girlfriend and fell for her. But I still think about my ex-girlfriend. I am not in contact with her, but I do think about her. My guess is, I'm not madly in love with my current girlfriend because I have been trying to replace my ex-girlfriend with her, which is a very stupid and selfish thing to do. But I don't know what to do. I could do anything for her(ex-girlfriend), and I still can. I don't if I miss our relationship more or our friendship more. I'd like to have our friendship back. I wrote a letter to her but haven't been able to send it yet. It's not a romantic letter at all. It's an apology letter for the mistakes I made during our relationship. I want to send it, but I feel I'd be cheating on my current girlfriend if I did that. I don't know what to do.