Keep your eyes on the prize. Stay in for five years, accept your lot but work hard, play the game, climb the ladder, specialise (important), enjoy yourself outside of work, leave at manager level for a good job on the outside with a solid name on your cv, and you're set at the age of 27.
Or, look at other grad schemes elsewhere, compare where you'd be after that five years (less speedy advancement and money in industry, banking, of course, not looking so hot right now). Might actually be more fun, less travel.
You're not in a bad position, quite probably just a bad project. If so, play the game and get on a good project.
If you go, don't jump out before you have something else (better) lined up.
Believe me, I totally sympathise, but that's the way it is at big4, you've just got to think of the long run, when all the grunt work becomes worthwhile.