I think you made the most valid point right at the end of your post. You have feelings over someone you haven't known in eight years. So you are not really imagining that person as they are now, you are holding on to the image of them as they were then. And eight years can change someone an awful lot, especially if those eight years were the transition from teenager to adult. But you will have also changed an awful lot, perhaps without even realising it.
I have been in exactly the same situation as you. I am a 28yr old male, and when I was 18 there was a girl who I was abolutely head over heels in love with. I would have done anything for that girl, and I believed we were soulmates. Then circumstances split us apart, and I spent the next 10 years feeling like there was unfinished business. Then I met up with her again and re-visited the whole thing. But you know what? The reality was that it was not the same, and that what I had seen in her in the past and had built up in my mind to be so wonderful just didn't appeal to me anymore. The only benefit that I got from seeing her again was to be able to put the whole thing to bed, and now I never think about her anymore. And I too am now engaged and very very happy.
I think that you should concentrate more on the reality of your current relationship than the fantasy of your past relationship. Don't jeopardise what you have now, your partner deserves more than that and so do you.
Take care...