"Do women really go around thinking "I know I'm attractive". (sorry, but that's the bit of your OP I find astonishing)"
"Good god are looks really that important?"
I find these comments disingenuous, at best.
There are tonnes and tonnes and tonnes of attractive women (well, people!) out there. Do we really think they're all pshhawing everyone away self-deprecatingly, totally oblivious to their own attractiveness? 
And as for whether looks are really that important, clearly they are, rightly or wrongly. Should someone's entire self-esteem be tied up in them? No. But you're living in some sort of parallel utopia if you think we've achieved some sort of state of being whereby looks are not important.
OP - I can relate to a lot of what you say. I was a geeky kid, tall and gangly and always being mistaken for a boy. I blossomed at the end of high school and spent my 20s being good-looking. I'm 38 now and even with all the added confidence and wisdom, objectively, I don't look as good as I did then, I just don't. And it's only going to get 'worse'.
However, I'm still happy with the way I look in spite of no longer having that glow of youth. :) I'm slimmer than I have been in years through hard work and feel so good for it. Initially after my 2 DC I felt invisible, as you describe. And I must admit, being that generic 'Mum' pushing a pram really does render you invisible, but it's not personal.
I agree with everyone else who's said you need to have something for yourself to gain some self-esteem from, and also to do esteem able things.
Here's something really simple you can do which I can guarantee will make you feel good. Pay other people compliments. Genuine, sincere ones with a smile that reaches your eyes. Do it often. There's so much that is complimentable about other people and it helps you to see that beauty comes in all shapes and sizes and forms. Try it and see what it does for them and consequently how it makes you feel about yourself. You'll get a lot of perspective about all sorts of things this way.