It's worth it. You did say you were an average student, and from doing the IB you got 34 (which is a little bit higher than the world average). How is this surprising? Your IB experience depends a lot on how your school handles it and how you handle it, i'm sure a lot of IB students felt the same way as you, i do at times.
However, remember that grades don't really mean anything until your predicted grades, this (December-March IB2) is the toughest months of your life, it's when all the IA's EEs, ToK and all that **** starts piling in. You have to pull through it, dont give up now, and it will all be worth it.
IB is hard and unis know that (top unis do anyhow), and if other students havt heard of it before, theywill realise how hard it is when they know you. Despite missing my HL offers (they wanted 36, 666, i got 40, 665), they still let me in (i'm pretty sure its cuz they appreciate the fact that i did the IB), if i got AAB in A-levels and they wanted AAA, they wouldn't have let me in.
Besides, you get to feel special when you introduce yourself and ppl stare at you dumbstruck when they hear how many subjects you had to study in your last 2 years. The time management and workload in IB allowed me to cope fairly well with Bristol Law (where they throw you in the deep end), while my friends are already missing lectures and/or not doing readings because they're not used to the workload.
Do your best, you're almost there. You've already chosen this path, make the best of it.
EDIT: oh yea, if your school didn't tell you, it's their fault: IB is a 2 year course, and by 2 years, i mean from day 1 to end of exams, you're not supposed to have a break, every holiday should have been used to work, if you managed to do that then there should be less problem.