Yes. I had a boyfriend my senior year of high school (I’ll call him #1), and after graduation, we spent a wonderful summer together and were deeply in love. Then I left to go to college, and he went into the military, and we pretty much had no choice but to go our separate ways…
I met someone else my freshman year in college (I’ll call him #2), and I loved him very much too, but never forgot about #1, and always loved #1 deeply in my heart. However, at one point I found out that #1 was married with children, and it was a disappointment, but I was with #2 now anyway. But I still never stopped loving #1. I spent a total of 9 years with #2, but that relationship eventually ended.
Then on a whim one day, I Googled #1, found him, and e-mailed him. We talked on the phone, and I found out he was divorced. The spark was still there between us. But he was living across the country, so we spent 6 months doing the long-distance thing, and then I moved across the country to be with him, and we’ve been together ever since, almost 4 years later.
So yes, it is definitely possible to still be in love with someone you have lost, even though you’re with someone else. Luckily in my case, I was able to reunite with my loved one in the end.
Almost the same exact thing happened to a very good friend of mine. High school boyfriend, her first love… they lost touch, and she married someone else and had a couple of kids… then she divorced and reconnected with her #1 via MySpace, and low and behold, they are now married.