Not sure what type of music you listen to, but there's a line in a song that carries the idea of: is it really him I'm missing, or is it my innocence?
Are you really missing your old BF, or the freedom you enjoyed at the time in your life when you were with him?
With a 20month old, a fiance, and I'm sure the run-of-the-mill living expenses and probably daily-grind job, life just isn't as fun as it was as a teenager and very young adult.
I get together with the group I used to hang out with in college and I have to laugh. We used to go to the bar every.night.of.the.week except Monday. We would drink some, tease each other some, spend time with the SO we were with at the time, play pool, video trivia, etc. We'd get together at a friend's pool and drink and BBQ and not worry about anything. We'd all pack up and head down to the lake to camp for a weekend of complete no responsibility.
Now we get together for 1st and 2nd birthday parties. There are always a smattering of kids. There's beer, but most people drink sodas, now. We talk about our jobs and families. We talk about kid's poop and sleep schedules. Everything is so much more serious now. We all enjoy it - but it's always funny to me to see this group now - to know how different we were 5-6 years ago.
You're spinning a fantasy in your head about your ex. You have this ideal image of him - and it's entirely possible he isn't anything like what you're imagining. And this fantasy is probably hurting your relationship with your fiance. If you can get past this ex, your relationship with your fiance will probably pick up more steam...
Maybe you need to talk to your fiance about doing more "fun" things. My daughter is coming up on a year and half and I'm definitely beginning to miss being able to just go and do, to take a quick trip with my husband, to do anything without the responsibility of planning for my daughter, as well.
(Don't get me wrong. I love my daughter more than anything, but she's getting a bit more independent - I'm just ready for a bit more "me" time - not a ton, just a little.)
Really think about what it is you really miss - him, or the carefree time in your life when you just happened to be with him?