How Do I Keep Positive Momentum Going?

You sound like you have the right ideas already---you just need to give it some time.
The number one priority to avoid depression is just to do what makes you happy.
it's that simple. It ISN'T selfish---a depressed wife/mother is no good to anyone. You must practice self-care first.
If your husband's family really makes you feel so horrible, then I would tell him that if he wants to be with them for the holidays then he should but you have to take care of yourself, and that means staying away from them, so he can go on his own. If they come to you, I would arrange to go somewhere else to visit someone, or stay but make plans to be gone much of the time (you can volunteer in a soup kitchen or something, or go for walks, take a book and go read in the park--anything to keep you away).

My favorite quote:
"I believe in pink. I believe laughing is the best calorie burner. I believe in kissing; kissing a lot. I believe in being strong when everything seems to be going wrong. I believe that happy girls are the prettiest girls. I believe that tomorrow is another day, and I believe in miracles."
-- Audrey Hepburn
I read this quote for the first time about a month ago and have been thinking about it ever since. I especially like the part about happy girls being pretty and I think that is so true, no matter what you look like. People just gravitate toward happy people. Happy people also tend to take better care of themselves. I am a complainer by nature, but lately when I feel like complaining I think about that quote. : )

Plan lots of family activities together outside of the other family members, travel, make dinner together, play alot and spend time outdoors. Volunteer if you feel the need. Get back your life and be strong so that when the toxic family comes around, you will have more fun with your own life, you sort of ignore theirs without getting pulled in again.

It sounds like you have decided to live life instead of living through it! Good for you! I have toxic in laws as well and decided long ago to stop letting their dysfunction destoy my postive outlook. Luckily, my husband feels the same way about his relatives. I really believe that finding a church that meets your needs is a great way to get involved with people who are in the same place as you are. PTA/school functions are a great way also. You will meet moms as well as teachers and school staff to get you out in a good mix of people. Good luck and you will be in my prayers. I will keep a look out for your posts!

Here's a Church that can take your life for positive turn, a change you won't regret:

http://www.newlifetabernacless.org/directions.php