I don’t think anyone is being horrible (including you). I do think you have a lot to learn about establishing and maintaining healthy boundaries. You are not going to be able to have healthy relationships until you do.
To start: You should not be your mother’s confidante about her problems with her ex. That simply isn’t your business. Tell her politely that her issues with her ex are something she needs to take up with him or with an adult friend. You don’t have advice to offer her. You are her daughter, not her adult friend.
If she brings it up again, simply tell her that you love her but you can’t help her with this topic and that you will leave if she persists. Then do it. Do it kindly, but do it. You can say something like, “I’m sorry this is so hard for you but I really can’t help.” Change the subject if you can. If she won’t let you, go to another part of the house or go for a walk. She’ll probably be annoyed at first but she’ll eventually quit asking if you quit responding.
It might also be helpful to establish regular household chores for yourself so that you do contribute but you’re not always having to drop everything to do what your mom wants. See if you can have a conversation with your mom about what you can do regularly to help out and still have some free time.
Beyond that: The problem is not that other people want you do do things or that they gossip. Small towns are just like that. The problem is that you haven’t figured out how to take charge of your life. At 20, you are at a time of life when there are many big decisions to be made. It’s normal to be nervous about it. If you are sometimes overwhelmed by the idea, you are not at all alone.
Do you think that maybe you go along with everyone’s else’s demands in order to avoid making those decisions? Or is it that you have an excessive need to please? Or maybe it’s a combination — or something else altogether.
Since I don’t have enough information to work with, I do suggest you talk to a local counselor. Apparently, your mother isn’t able to provide good guidance so you need another person to be your sounding board. Yes, I know that going to see a counselor may feel like just one more thing to do. But it’s very possible that investing that hour a week in yourself will help you find some direction and set some boundaries with others.
I wish you well.