A pants conundrum...
 

So I got a pair of jeans today that are most definitely too small. They are the same size as the pair preceeding them, but I've had the old ones for a couple of years and they have stretched.They are not, however, undoably small. As in, if I lost about 10-15 pounds, they would fit superbly. Should I return them or use them as inspiration to work harder? I worry a lot about setting myself up for failure or disappointing myself but I DON'T want to have to face a larger size either...

Personally, I would return them and buy a pair that fit properly and make you look great at the weight you are today :) You'll feel good in the new jeans and perhaps that will give you the confidence to keep going.
I used to feel the same way and would squeeze into smaller sizes, creating a muffin-top. Buying the next size made me look so much thinner so I stopped worrying about the size and just got what looked good ;)
Perhaps the small jeans would motivate you, but I think they'd just depress me since it'd be another pair of jeans I couldn't wear.

Do you need them now, or were they just a random purchase? If you need them, I'd swap them out for some that fit now. I agree w/ Sarby on that point. How they look in the mirror and how they make you feel is more important than the number on the tag. However, if you aren't really in need of a new pair right now, I don't see any harm in keeping them to keep you motivated. I have a bunch of jeans that haven't fit in anywhere from 3 to 9 years. In the past few weeks I've been able to start wearing 2 of the pairs from about 3 years ago again. And that feels great! To think that just a month ago they didn't fit. So, if you can afford the extra not-currently-useable purchase, I'd keep 'em and use 'em to measure your progress and as a reward for when they do fit. :)

If you have other pairs that you can comfortably wear now, I'd hang on to these pants as motivation.

I know how much it sucks to have to buy a size up from where you have been convinced you are. So if it feels too much like a sucker-punch, you c ould try taking the jeans back and looking for a different brand or style. I don't know about you, but I know I wear different sizes depending on the cut and brand of the jeans. Maybe it's just those jeans that you have to go up a size in - which means they're not right for your body type. Look around and find some that are perfect for you. It'll make you feel thinner if you get the right style.

I know I wear a 12 in Seven jeans where in a lot of others, I need a 14. What does this mean? It means I pretty much only wear Seven jeans. lol ;D