It's done! The bag, I mean. I finished it 5 minutes before we left to go to my mom's birthday and it doesn't have an inside pocket yet (so, yes, I know, that means it's not 'done'-done. Shush.) but I'll add that later.
Here it is!
I folded the lining fabric over the edge and sewed it together that way, because I'm a noob and can't measure a square piece of fabric. I had a bit too much fabric left at the top... Sewing the lining together was a Bitch, but together with the bf, I managed to get a sturdy lining. The handles gave me a headache (did I mention that for some reason, the yellow-pink-red one was much longer than the green-blue one? And that I have no idea how that happened?) and the bag itself rekindled my RSI, but it's done, and my mom is very happy with it. I'll call her tonight to see if it survived her taking it to work yesterday.