Ohhhh, I can relate to this! I think part of the feeling is down to the fact that as single people our lives were outwardly focused on the world. I longed for the security/intimacy of my very own family unit. Now that I have it, my life is inwardly focused....and by that I mean, focused on the people who inhabit the four walls of our home, and what transpires there. At it's best, it's a cozy little world, but there are times when it can feel suffocating - especially when those people are behaving in a less than edifying way.
I think perhaps for you the freelance world could also be a double edged sword. It gives precious flexibility, but means that you are in those same 4 walls much more. There is little chance for a break, or change of scenery.
When we were single, if a friend acted moody/strange/terribly, we could take a break from that person. Now, we spend every waking moment with the people who depend on us. It can be wonderful/charming (or even winsomely helpful!), but also is never far from mundane. If demanding/unreasonable behaviour is added to the mix, it's easy to feel guilty, angry, frustrated, bewildered, fed up. For example - while I'm writing this, dd is behaving quite nicely, but she still has interrupted me 3 times with various requests. This being a parent lark is relentless!
Anyway - this is a long, rambling way of saying that I think it is important to do things for ourselves apart from the kids. And I'm not talking about time to go to the hairdresser or dentist. We need to make time to go to the movies, have a meal with a friend (without kids in tow), see an exhibition, etc. I know I am guilty of running out of energy when it comes to planning things for myself - it is always the disposable part of my schedule. And it is easy to feel knackered, floppy, and devoid of any "spark". And I know completely what you mean about plummeting self image and self esteem. I used to have a wardrobe full of gorgeous clothes. Now I no longer have the body/budget for it. And my priorities are different. But, I think we've allowed ourselves to disappear as we've gone about the business of trying to turn ourselves into successful mums.
I think I should take a uni course, or open university for mental stimulation and satisfaction, but I fear that it will become another chore and another thing to feel guilty about not doing as well as I should.
So, I don't know what the solution is. Like you, there's nothing awful going on (and that is a huge thing to be thankful for), but I feel I've lost my sparkle. So, I'm right there with you in feeling the same things, but also don't know how to change it. DD just looked at me and said "mum, let's go do something. I have loads of energy - like a rocket!" And, I'm thinking I'd like to take a nap.....