If you possibly can, I would get a new - or used, by all means! try ebay? craigslist? can you borrow one from a friend? - and smaller pack and let the Camino show you where you can find that money. Staying in the albergues, sharing a cooked meal in the evening, one less café con leche here, one less glass of vino tinto there, and you'll soon have made it up. It doesn't have to be expensive as long as it is smallish, about 35 litres, and you only bring the bare essentials. The pack you've got doesn't seem to want to come with you. Maybe it isn't meant to. I have heard wonderful things about REI, why don't you go there and see what they can find for you? Bring your gear and try the packs in the shop with your kit inside, so you know there is enough room and that it feels good. Search this forum for 'packing list' and you will find many good ways of reducing the pack weight! You still have time before you leave, I hope you have time to decide what to do without getting stressed out. It's the Camino. It will be fine.
Buen Camino!
Linda