Can you love two people at the same time? F(26)

Not sure how to best explain my situation, but here it goes:

I met a guy when I was studying abroad and was 22. We had a brief summer romance that did not last longer than 3 months. I had to go back to university abroad and he stayed in Austria.

We rekindled 2.5 years later while he was in Singapore and I had moved to London. We spoke every day on the phone and he flew to visit me in London. At this point I was 24, and while I enjoyed it, I let the communication die and ended up being quite nasty and ghosting him. In hindsight, I do not recognize myself with behaviour and regret how I acted deeply. I really do not know why I acted this way- the only reason I can think of is that I was in my phase wanting to be single and freshly moved to London.

Flash forward to now: I have a loving, kind boyfriend who I have been dating for 4 months. It feels quite serious and he speaks about wanting to move in with me (even though he is staying in London), and wanting to get married. We tried moving in together for a week and I really hated it, but that is a different story. I really was not thinking about my "ex" too much apart from feeling sad and guilty about how things ended and wishing I had acted differently. But now this has changed- I find myself thinking about him everyday, fantasizing about reaching out to him and wondering what if, wondering if we still stand a chance.

This makes me feel super guilty towards my boyfriend. If I were in his shoes and knew my partner was thinking about someone from their past this much, I would be furious. On the other hand, I really cannot stop thinking about my "ex". Additionally, my bf is staying in London and I am moving to Munich or even Vienna (where my ex lives- this is also where my family is from).

Why is this happening? What should I do? Do I just accept that my "ex" is the one that got away and get on with my life? Or should I risk sending him an apology letter? What is triggering this? Any ideas welcome. Please be nice : )))

TL;DR: I have a boyfriend but I regret how things ended with my ex and want to reach out as well as fantasize about rekindling the romance. Is that emotional cheating?