The easiest way to slow down your EXP gain is to pick up a second class and use it while you're running quests.

You're level 50 so (assuming you have the expansions) you can go unlock RDM or SAM now and start putting EXP into them.

If you hit level 60 of the MSQ and you've still got an issue with too much EXP on your main class, grab GNB or DNC and spread it further.