You and your husband have to talk without your children around about your family choices. Hire a babysitter if you have to and get out of the house to talk. Remember your joint decision should be what is best for your family, you didn't just have a baby, both you and your husband have a family. If you jointly decide for you to stay at home make the most of it for both you and your girls.
How to keep your sanity and feel like there is more to life than your kids?? I am an extrovert who gave up an 11 1/2 year career (with a Master's degree in Engineering) to stay at home and make a difference in my kids life. Last year my oldest son's Kindergarten teacher told me I was her most overqualified volunteer!!
Volunteer--and make sure you put it on your resume to keep it up-to-date. It will help 'fill in' those gaps until you go back to the full-time workforce!
Military posts (Ft Stewart/Hunter Army Air Field) provide opportunities to volunteer and pay for childcare while you volunteer if your spouse is military (active duty, national guard, or reserves).
Local schools--they need volunteers in all sorts of things and some of them will let you bring younger kids with you. Your kids get used to being in a school environment and there are other volunteers around for adult conversation.
YMCA--Join the local YMCA. Most have free childcare available for members when you work out or want to just read a book while at the Y. This is a good way to get 'Mommy' time. Be a YMCA coach for your girls. What a great time I've had, and I didn't know a thing about coaching the sport!
Playgroups--There are numerous nationally affiliated playgroups available(MOMs, Mothers & More, or Mommy and Me!) If you don't want to pay national membership fees do a google search of your city on the internet for local playgroups or form your own playgroup. Start with parents/kids that go to your church,your local libraries storytime, or your local parks. I can send you an easy to use template of one that I use if you would like it.
Check out the local churches around you and see if any have an aerobics/exercise bible study available. In Texas there were multiple churches around me that had Wednesday morning sessions that were 1 hour exercise followed by 1 hour of bible/book study (Catholic, Methodist, and Baptist churches). They had child care on site for the kiddos. I had a 2 hour break with other adults and my kids had structured playtime with friends. Win-win.
Although I miss having a paid career for these last 4 years, I've enjoyed going on those field trips and helping with class parties. My son actually likes me volunteering every week for his teacher and I always know what is going on at school.
Good luck with your choice!