I can definitely relate. I was a single mother for 16 years. My child is finally grown!
It is a very difficult task to raise good children and keep yourself sane. I know sometimes you just feel like running away and have a good time, and that is normal. I hope you are doing some adult things to have fun. Schedule a babysitter and go out with friends at least once every two weeks. Do some things for yourself. If possible, use your work time as a social outlet as well. I am glad you continue to take your medications to help you.
As for the children, I hope I can be candid in saying, your children are behaving the way you teach them to behave, unless they are suffering from a childhood disorder such as ADHD. When you say "they won't take no for an answer" it usually means you do not follow through and apply consequences for behaviors. It is very important for children to have structure and consistency. If you have allowed them to sleep with you before, and maybe just too tired to make them do something else, then they have learned that if they beg long enough you will give in. And this is true for many things, if you consistently give in. This gives you less time for yourself as well.
There are lots of good reinforcemnt ideas you can use to help. At that age, children will usually work for something tangible. I used a wekkly poster board with certain things on it that were age appropriate, such as brush teeth, pick up toys. sleep in your own room, etc. Each day we put stickers in the place where the things were actually completed. At the end of the week, a certain number of stickers earned a prize. This was usually something that made it productive to work toward, such as going skating, or a certain inexpensive toy. Just be sure to follow through with the prize. Once the children learn this is how to earn rewards and these are the rules it helps them correct their own behaviors, without you telling them.
Now, for you, it is important to accept that you are stressed out and may need to talk to someone about how you feel. You know it isn't healthy to cuss and yell at the kids, and blame them for being alone. But, I also know how frustated you may be, and overwhelmed. You may want to talk with a counselor to help you feel more able to take care of your own emotional needs. Don't beat yourself up about this. Just work to feel better and help your children to feel better.
There are many web sites available to help you with the parenting ideas and to offer support.
Here are a few:
Pleawe let me know what other questions you have. Gina