This happens to me all the time. This is how I handle it. Once I finish the OMGFUN!!! scenes I move to the slightly less ecstatic but still Fun! bits. I happen to enjoy writing dialog, so I'll get two or three lines of dialog pop up in my head and I write them down immediately and try to work them into a scene. A little more work but still fun. Figure out what you enjoy writing, if it's dialog or character sketches or kissing scenes and write as many of them as you can. Arrange them in chronological order and stare at your fic. At some point, you'll reach critical mass where your fic is not just a bunch of unrelated scenes but you can see the story line tying them together. Your fic is now officially an Unfinished Story.
Here's where my secret weapon comes in handy. I anthropomorphize the fic. It's my baby and I want it to live. Babies are cute and all but no one wants to deal with vomit and diapers. You do it because the baby needs it.
If you don't, the baby will never live. It will never grow up and leave the house and make you proud. It will never have a meaningful relationship with a reader. It will never come home proudly showing off its first little feedback. *sob* The guilt! Bad mother! Bad! It's time for tough love.
I'm writing about it facetiously but at some point the story has enough personality and energy that it begins to demand completion. And it will nag you and nag you and nag you... This part of the writing is Not Fun. It's nasty, tedious, slogging work and accepting that it isn't fun is the only way to break through that barrier.
But as my momma always told me, "Once your baby is in your arms, you forget all about the agony of child birth." o.O Yeah, I don't believe her either, but in the case of writing, it's actually true. Or we'd never ever sign up for another ficathon.