As long as you eat very healthy, mostly fruits and veggies and rice and beans and all the other yummy healthy nutrient rich foods, you'll be fine! You can even buy a pregnancy food book that tells you what you should or shouldn't eat. I have a friend who gave birth to two children and you would never know it because her body is amazing, and just look at all the celebrities who go right back to their original size after a month (lol). It's definitely possible. You won't ruin your body don't worry. Eat as much as you want though, because you're eating for two, so don't try to stay skinny for that reason, just eat healthy portions and healthy food and drink lots of water and do very light exercises and you'll be fine.
As far as the stretch marks, rub lotions and body butters and oils on your skin every day of the pregnancy and after. This actually makes a difference.
This is a time to feel gorgeous and beautiful and glow. Enjoy it its a beautiful time for your body. Just knowing your body can harbor life and do such amazing things should boost your confidence, not bring it down. But of course don't try to stay skinny because this can cause harm to the baby by being born underweight but I'm sure you already know this.
When the baby is around, just put the crib next to you while you work out in front of the tv, that's what my sister did. Her baby was in a little gated baby crib playing with toys while she worked out in her living room. Your sex life definitely hasn't stopped after having a baby, my sister's husband also wasn't around for their baby and she has had like 2 boyfriends since then and they had their own kids too so its not a big deal. Why isn't he offering child support? Can you take him to court for that? Can you ask his parents for help with their grandchild? Don't feel guilty for getting a babysitter. Do a water bath if you want during the labour, the water is usually luke warm and it helps because its a more calming environment to be floating in a tub. The doctors have drugs for the pain. And guess what? Being a single parent can be a really good thing because you get to raise your baby specifically how YOU want, without having someone else raise them in a different way or fighting over parenting styles.