What you've mentioned seems perfectly normal. Don't feel bad about it at all! At least you're not letting your feelings get in the way of your better judgement to spend as much equal attention to each as you can.
And YES, most certainly a dog at 10 months can get depressed. She probably already was at one point when she first left her breeders house and she just bounced back faster than she did after coming out of her heat cycle. For some females, those heat cycles can be real tough....cramps; headaches, many if not all of the same symptoms you or many other women you know experience during that time of the month. Luckily for these little guys it doesn't come that often (or shouldn't!).
Getting her spayed sounds like a sound choice. (that sounds wierd to me coming out of MY mouth) From what you've mentioned so far, it sounds like she'd benefit greatly from it at such a young age (to me, spaying under 16 months is young). Just watch out for excessive weight gain afterwards.
Keep that chin up!