I had loose skin leftover from pregnancy and I've always bordered between being the low side of a normal weight and underweight. Mine did tighten up a bit over the years after pregnancy but it's still looser than it used to be 6 years on.
Stretch marks - I got them on the backs of my legs, my hips and breasts during puberty. I never gained or lost excessive amounts of weight in that time, it was from growing. I was horrified by it and it was a long time before I would wear anything that showed my legs. During pregnancy I got stretch marks all over my stomach as well as on my bottom (I think...I haven't seen them but I can feel them). The ones I can see have faded now but I can still feel them.
My friend had a lot more of an "overhang" from her pregnancy and she has had a tummy tuck now and she feels better about herself. If that's an option you wanted to take, don't think that you can't but equally don't feel that you have to.
You're blaming yourself for something that you really shouldn't be blaming yourself for. It isn't nice to see our bodies change from pregnancy. I know some women embrace it, I personally struggle with it. The worst part for me is my breasts. They're two different sizes now. I can't find a bra that fits them because of that and I hate them. It does help (me, anyway) to think about the amazing thing your body has done. Growing and nourishing a new life. Of course it will take a toll.
When I see women who don't give a shit about how they look, I secretly feel jealous of their confidence. It also makes me try to be the same way... it kind of inspires me (yes I am lame). I'm far too paranoid and insecure and I think it shows. Every now and again I have that "I don't give a shit" attitude and I feel so much better for it.
Also, sorry this is a long reply, but something I am trying to learn myself and I think you could do with learning, is that your body is your body. It isn't something you have done wrong or should be blaming yourself over. It's just your body. You don't owe it to anyone else to look a certain way. If it's unhealthy due to a bad lifestyle then you owe it to yourself to keep it as healthy as you can, but beyond that, how it looks, it's just how it is. I hope that makes sense. It's kind of what an ex said to me once when he realised how much I hated myself, and it helped me a bit. What's also helped me is having my DD. To me she's perfect and I'd be devastated if she had the feelings of hatred about herself that I have about myself. I don't want her to grow up thinking that way about herself and I would be so upset if I'd "passed on" my own insecurities to her.