I’m going to ignore the fact that a 27-year-old dating a 19-year-old is a creep in my response, but FYI, that’s a major red flag.
I’m not totally sure I followed this, but it sounds like you dated this guy briefly at 19 and again briefly a few years later, and have since been friendly when you’ve run into each other over the years. You’ve even gone so far as to tell one another in recent-ish years that you wish you were still together while both of you have been with other people (SUPER unfair to your partners, btw). It sounds like this is someone who should stay blocked. You have your reality of your current life, which I assume is imperfect as real life tends to be… buuut you’ve built a fantasy around a man you care(d) about. You need to learn to drop the fantasy. It’s stopping you from enjoying the life you actually do have.
I think it’s fine and normal to remember the people you’ve dated who made an impact on you with fondness and love. I know I have a special place in my heart for some of the men from my past, even one I only dated for ~6 months. And yeah, I guess maybe in theory things could’ve worked out with them if one or two things had been different. But things unfolded as they did at the time for valid reasons. But they’re not my reality.
Therapy would be a good move for you. You can unpack the past and maybe figure out how to make your reality one you enjoy enough that you don’t spend this much time wrapped up in the fantasy of what could’ve been with some sketchy guy.