Hugs Sally.
You are not the only one with the isolation problem. Many of us have to turn down invitations for fun things because we have to be home, since we are the only responsible parent in the house. It's hard to watch other people go out and do fun stuff when you feel like you live responsibility 24/7.
Try giving yourself some small moments of fun. Take the hour after the kids go to bed for some down time for you. A bubble bath, listening to some music, reading a good book...anything that is all about you. Then incorporate other fun times into your day. A special lunch, a walk after work, calling a friend for a few minutes.
These things will do a world of good for your spirit.
From small things, big things one day come.
Peace,
Gabe