Waiting, you're young yet. Your youngest is only 2. I didn't get properly stuck into my career until my youngest was 16.
I remember feeling like this when I was a SAHM. I was so unused to having proper conversations with adults that I became tongue-tied in social situations. I even forgot my address once in the panic of being asked where I lived when I wasn't expecting the question!
It's even harder for you, because of your health condition. Don't be so hard on yourself. Are your children happy, loved, well-cared-for? That'll do for now.
My advice would be to develop one or two interests of your own. Reading, crafting, baking, for example, are things that are easy to manage when you are home with the children. Maybe join a group or class. Are there any support groups for people with your health condition? Or a parents' group?
You can add something to your family's lives just by being there for them - being kind, warm, reliable, sympathetic. You can be valuable to society by bringing up your children to be loved and loving citizens, by supporting family and friends, or, if you want to extend your reach, by doing some voluntary work.
Try not to compare yourself with others. It's not helpful and, in truth, you don't really know the reality of their lives.