I keep quitting RP and coming back to it, I'll admit.
What I've found is just carve out a time each day that's all about writing (no AIM, no journals, maybe only a chat room that's ALSO focused on writing, not chatter). Sit down, do two thirty minute word sprints (write for thirty minutes, no stopping, no editing). THey can be one in the morning, one in the evening, or both at the same time with a five minute break to stretch, get water, etc. in between.
Then the rest of the time is yours for RP. But as long as you make words every day, you'll be fine.
750 words a day gives you a 90k novel in three months. 500 words a day takes a little longer, but is just as good.
And remember to compartmentalize. Take the time for each and don't feel guilty about ignoring RP for an hour so you can focus on writing. If you want to write, the only way to do it is to write.
It's not easy. It's actually horrifically hard, honestly. I've gone back and forth between my RP, my fic writing, and my professional writing a lot. But it's possible. I have two short stories coming out this summer in anthologies, and I keep fighting with my novels and hunting agents. But the big thing is to set aside time for yourself to write.