The thing I usually ask is, if your hair is long enough to be pulled back, are you pulling it back more than half the time? If the answer's yes that's a mark toward cutting it shorter.
I think no matter what, hair's high-maintenance- you have good and bad hair days no matter what lengthy it is. (Unless you're bald, in which case, good or bad scalp days?)
This is good advice.
Fiona, you seem to have a round face, so the cut is really important. I have a round face too, and have only had my hair really short once. At the time, I liked it, but I look at photo's now and think how much I looked like a boy! Mind you, I was only 12 at the time!
If you still have fairly long hair, why don't you experiment with something mid way first? Like something shoulderlength and layered around your face? That's what I did in my mid twenties, and it's what I do now if I get the shits with having long hair.