I think it's stress, honestly. We get so busy, between work and its demands, racing home to be with the kids, doing everything we can for them, buying groceries, making sure they get to where they need to be, feeding them, cleaning the house, doing the laundry. . .. at some point in there it's easy to find your selfhood evaporating. You get to the point where life is all about them, and you feel basicall invisible, and you don't feel like the person you used to be.
Guess what? you aren't. You are a much richer, much more capable person. Trouble is, as moms we don't do so much for ourselves, and we do so much for the family that it's hard to feel good about us.
I think (and I could be WAY off, so just tune me out if I am) that it might be helpful to do something good for yourself. Go to a gym and start an exercise plan, or go to the Y and start swimming laps or something. Do something that is GOOD for you, and helps you to take pride in a new accomplishment. Take an adult education course in something you'd like to do -- our local school offers all kinds of stuff -- line dancing, quilt making, cake decorating . . . and it's a good way to meet other people, as well as to accomplish something new. It might be tough to juggle your family time to find time for just you, but if it leaves you feeling proud of yourself, and also healthier (in the case of exercise), then your family will be happier that you did it.
Also, look at whom you are measuring yourself against. Is that person working FT AND juggling 3 kids and a spouse? Oftentimes single people seem so much more "out there" and interesring in a crowd, because they do it a whole lot more than we do, and because they HAVE TO, in order to have a sense of belonging. You already have that sense of belonging, and I do, too, but I do find that in social situations, I have more to say about my kids than about myself, and I'm sure no one wants to hear all that !! Proud as I am. oopsie I do some other stuff, like sail on tall ships, but that's such a foreign world to most people that while it's interesting, they don't want to hear about it either. In fact, most people don't want to hear about anyone other than themselves -- so if you can come up with some "token qustions" to get conversation going, you won't have to talk at all. . . . That will at least get you started.
But, really, do something for you I suspect you are doing so much for the rest of your family, and for work, that you've forgotten what it's like to do something for yourself. And the first step is to give yourself permission to do something special, just for you. it will enrich your life, and that will enrich the lives of your family, as well.