You need a break! Two kids (three kids!) that young is a huge amount of work. See if you can find a "mother's helper" type girl. Maybe not even old enough to babysit alone, but old enough to be helpful to you. She could read to and play with your dd, play with the baby, etc. I did this a few times. It's nice just to have someone you can talk to that converses in complete sentences. You wouldn't need to pay very much, as you would be there doing the bulk of the work, she would just be "helping." You may even find a girl to do this for free, if she's getting close to the age of being able to babysit, it's be a great way to get experience without the total responsibility. You could check with a local church or something if you don't know any young teens.
Also, and I don't mean this to sound accusatory at all, but I must agree with the poster who asked why your dd was alone. It sounds like she isn't ready to be left alone at all. You must watch her all the time. Yes, it's very hard and it means you have no time to yourself. This is why you need someone to help you sometimes. You may want to look into a Mother's Day Out program as well. Many churches have these, usually one day a week for a few hours. You don't have to belong to that church or anything. It helps when you know you're going to get a break. When it just goes on and on with no end in sight, no wonder you feel overwhelmed. I adore my kids, but some days I can't wait to go to work, so I can have some peace and quiet (and go to the bathroom ALONE!) and I work in a Level 1 trauma center, not exactly a peaceful place

(It's a lot like ER, only without all the sleeping around

) What I'm trying to say is, it's more work and more stress to take care of young children than to work in a trauma center. Don't feel guilty because you can't do it all 24/7. It's exhausting.
Also, I've known a couple of kids that did the poop smearing thing. I think they just liked the texture

They didn't have any underlying "issues" or anything. Have you ever made homemade playdough? It's easy and fun(and also edible so you don't have to worry). I'll bet your DD would love squishing the playdough

Yeah, if you have carpet, playdough is a pain. We have to keep it in the kitchen. But it's a fun activity. Also try fingerpainting, or just spray shaving cream on your kitchen table and let her smear it around.
There may be preschool activities at your local library or bookstore as well. Fun for your kids and you get to be with other moms. You may find it's even more helpful to vent to people in person that to us here on the DIS. You may find a friend you can go to the park with, etc. At that age, playdates are for the mom's sanity not for the kids' social needs
Good luck!
Laurie