First of all, I would argue against your assertion that your creativity is in limited supply and that when you exercise your imagination through role playing you are somehow diminishing the quantity of creativity which you can apply to your writing. In my experience, creativity is a muscle. The more you use it, the stronger it gets.
What are in limited supply are your time and your attention. If your writing is hungering for more time and attention than you can give it, that is great. It means that you have the bug and that no matter what happens from here, you are a writer. The addiction to craft stories is incurable, so get used to being frustrated about not having as much writing time as you desire. Add a little discipline to your time-management and find a way to feed (though never satisfy) your need to write.
It may be that what you are experiencing is purely attention based. Every time you engage in role playing, you become focused in someone else's creation, that of either your game master or the game's publisher. It could be that your current story is jealous of the attention that you aren't spending on it. There are times in any writing projects when I have to put down my nightly novel habit, fall behind on my tv shows and give every free moment to the story I am crafting. That is the nature of being in any relationship. Sometimes they are demanding and sometimes they are not. Again, this is a good thing.
It sounds like you are just starting out as a writer. In that case, practice writing is probably more valuable to you than focusing on your far-from-complete first novel. Short stories are a great way to start, but their brevity offers little opportunity to practice plot-development and scene-organization. Your role playing doesn't suffer from that brevity. Most quests go on for months and contain more details than can possibly fit into a single book. Why not practice your writing by becoming the unofficial chronicler of your role playing adventures. After each role playing session, scribble down the major events which occurred as a brief outline, then arrange that outline into a scene list and then flesh out each scene with artfully crafted prose. What you end up with, may not be publication quality, but it will exercise your writing skills and may exorcise (at least temporarily) your need to write.
Not everything which you write needs to be bound for publication. Everything you write, if crafted thoughtfully and carefully, will ultimately make you a better writer. ...and the same is true for creativity in general. The more you use it, the stronger your imagination will become.