I think this is one of the hardest thing to gain control without some form of help. You can try this. It seems that you lie about present stuff that's not bad in the first place to admit. I don't know if I understand your post correctly, but it seems a friend asks you to do something, and you agreed then you didn't have time to do it, but you still told her you did? If those are usually the situations you find yourself in, what you can do is write down everything you do, like a resume. For example, you can write down, went to grocery, didn't send out package for my friend yet, did half of my homework, and then you have the list out somewhere that someone can see it, and you can read it over and over, then when your friend asks "ohh, did you send out my package?" you can remember the list and say, "no, i didn't have the chance. I was so busy today. I'll try to get it out when I have the time, or if you really need it then can you help me with it?" ("cause now you have it written down in pen on a paper, and someone saw it too so it may be easier to say you didn't get it done)
For the issue with your able to not say no, I think it'd help in the beginning if you surround yourself with people who are already in your life that you know if you say no to something, they're not gonna get mad, or leave you. Do you think it's due to your fear that if you don't make people happy and do what they want they'll go away and leave you? You just have to think, that there are things that can compromise, and if you feel you're not ready to completely say no, you can compromise with people on what they ask you to do, and ask them to shoulder half the responsibility.
I don't know if I'm completely off point here, but I hope I've helped somewhat.
I really hope things will get better.