My short answer to your question is "No." People don't work on changing their behavior because I say something to them.

I think you're a little confused about what boundaries are and how you set them. Telling your friend you want her to stop breaking plans she makes with you is not a boundary issue, IMO. Your friend isn't breaking boundaries. She is being unreliable. That's a different kind of problem.

On issues where you do need to set boundaries, that's more about you changing your behavior than you telling other people how you would like them to change theirs. Assume that people aren't going to change because, generally, people don't. One part of boundaries is you controlling the access people have to your time and attention. Next time your friend starts talking about something great the two of you could do together, accept the fact that she may simply be fantasizing out loud. Show little interest. If she asks why you aren't showing any enthusiasm for her great plan, you could tell her that you've learned that she often doesn't follow through, so you are not taking her seriously. On an issue like this, "confronting" really doesn't accomplish anything. Your friend actually has a right to be unreliable. She's probably that way on a number of fronts toward everyone she knows.

Having boundaries doesn't mean you get to tell other people how you expect them to be. However, you have the right to decide how you will be and the right to set consequences you will impose when someone let's you down on something they agreed to. At the mildest level you can simply tell the person that you were very disappointed by them and guilt the heck out of them. This is not an act of "confrontation." This is giving people meaningful feedback. Then, the next time this friend wants to do something with you, make her practically beg on her knees. Keep reminding her that you don't trust her to follow through, given her track record. Sometimes, simple complaining enough will motivate a person. In Yiddish, there is a term: "kvetch." Sometimes, you have to practice the art of kvetching. It can be effective, especially with people who actually do care about you and don't want you to stay mad at them.

Lose the word "confront." It has become a cultural buzzword that people think connotes courage and bravely tackling an issue. And people are under the delusion that they can solve all kinds of problems by "confronting" others. A confrontation is often a climactic response to someone acting poorly over an extended period of time. Things build and build, and you erupt in a confrontation. Sometimes, an ultimatum is involved. Seldom is this a solution.

I once had a neighbor who thought it was okay to enter my apartment without knocking. He thought our friendship was such that he could just walk in. That's a perfect example of a gross boundary violation that does call for immediate confrontation. When I suddenly found this guy standing in my kitchen, "I said, "Don't you ever, ever walk into my place like that again! I don't care if the door is wide open, you knock and wait to be asked in." I spoke sternly, and he never did that again. I didn't make any threats, but I fully intended to call the police, if he continued to do that. Even without me saying that, he apparently knew I was dead serious. People can tell when you are merely hoping they will change, as opposed to when you absolutely won't tolerate something.

You've done a really good self-analysis, and you do need to learn assertiveness skills. I used to be somewhat of a doormat, myself. Learning to not let people take you for granted is a process, but it can be learned. When someone does you a wrong/incourtesy, think about what you can do in terms of your behavioral response, rather than how you want to tell them to change.

People don't change because you explain to them that you would like them to change. People change because they want to avoid a consequence. You have to give people consequences.