remember lots of the advice you get here are from people who have no concept what a hill really is...

for my moderately hilly commute (one steep hill on the way home) i run a 44x18 and seem to do ok! but when i get out of my element into the bigger climbs i just pull over and walk them when i can't pedal anymore.

singlepseeding will make you a better masher. so after running the 21 on the back for a while you might be able to slowly start going bigger. or you could start a little higher and resign yourself to walk a little till you get stronger.