That's it exactly!
I lack boundaries....I have known that for a very long time. It's like I either ignore my own boundaries or go way overboard defending them, and then people don't like me because I over-react. I overreact because it's like a volcano building up...and yet it's ME who is the problem.
Anybody have any idea how to assert oneself without being selfish, pushy, rude or aggressive? How do you set up appropriate boundaries and enforce them? Oh, dear me, I don't even know where to begin...that's why I shut myself away at home ... I think I gave up trying to figure it out ... but I'm so terribly lonely. Does anybody know of any good books on the subject? Maybe I could start there?
But, someone please give me a hug. I think I need one. I don't know how I will ever fix ME. 
Oh, the volcano!!! That used to be me!
HUGS!
For me, as I imagine for you, it started in childhood. I wasn't allowed to express anger. I was expected to be a doormat, and to be polite and keep my mouth shut all the time. Anytime I defended myself, I was told I was "too d*** sensitive" or "overreacting."
But, I had some close friends who would tell me when I erupted like a volcano "You should have told me at step A that you were angry. Instead, you let A fester, and B upset you, and C happened and you didn't tell me til we got to N and now you're livid."
So, I learned that with
some people, normal people, respectful and decent people, (as in
not my parents) you can actually calmly say, even at event A or event B "Hey, I'm upset about A and B" in a calm tone of voice and they will not yell at you or tell you that you're crazy or full of drama.
Now, I usually let event A slide. Whatever the first thing that upsets me is, I let it go. But when the second thing happens, that's when I say something. To normal people. To crazy people who overreact, well, I cut them out of my life, because there are actually a lot of fairly normal people in this world and life is too short so spend time with the crazy ones, even if they are related to you.
When I'm upset about events A and B, I usually compose an email or a letter. I'm not good with confrontation in person or on the phone. I can't think quickly and I feel panicked and backed into a corner and then I don't say everything I wanted to say, or I shut up because I'm afraid to get too angry.
So, I use email and the written word. Then, I can compose something and think about it for about 30 minutes before I hit "send." The best way to do things is to be factual about what happened, not accusational about the motives you perceived the person had, and then state your feelings and reaction, and then what you would like to happen to resolve it.
For example, if your friend has a habit of canceling on you at the last minute to do something else. But you still value the friendship and you don't think it's because she hates you and never wants to see you again. You would compose:
Hi friend,
I just wanted to say that I'm a bit upset. <- do not add "with you." Just say "I'm upset."You and I had plans to do X. And then you cancelled at the last minute. I understand that things come up, and it's not for me to say that your new plans are less or more important than X, but I was annoyed because I had to borrow a car, get off work early, change clothes in the car, etc <- give some examples of how it inconvenienced youI didn't say anything, because stuff happens. But then the next time we were supposed to get together, we were going to do Y and then you cancelled on me again. <- still factual. You are retelling the eventsIt inconvenienced me again. Again, I understand that stuff comes up. But now, it's becoming a pattern. And I feelAdd how you feel. It doesn't mean you're a poor me victim. But you don't want to accuse and say "You are a thoughtless moron." You don't know why your friend keeps doing this. But you need to show that you are upset and that if it happens again and again you're going to blow your top. Even just saying how you feel will let some of the pressure out of the volcano. If your friend is remorseful, you'll feel better. If she is not, you know not to continue to events C, D and E until you lose it completely.
So you are saying how you feel:I feel like I am not a priority. Like my time and energy doesn't matter. I feel like I am putting more effort into meeting you than you are. This makes me angry and doesn't make me inclined to make more plans with you in the future.And then say what kind of resolution you would like, if you can think of something.
Obviously, you want an apology. If that's all you want, then don't say anything more. Just sign your name. But if you want something like repayment for tickets you spent money on or something like that then add:I was out $30 for your concert ticket. I couldn't find anyone to go at the last minute and didn't want to go alone, so I'm out $30 for my concert ticket, too. I'd really appreciate it if you would at least pay me back for your ticket.And then the ball is in their court. But at least you've said your thing.
The only book I can think of that might help is
The Dance of Anger by Harriet Lerner. It's more about families, but it basically tells you how to just not engage in the fight or argument. How to just step away when you see a pattern. I'm sure there are others at the library. I usually look on Amazon because there are reviews and they are really helpful.