Boundaries? What are those.

So I just finished the book adhd and marriage, (and I would like to add I am not married but am in a 7 year long relationship where we have three kids, 2(ages 2 and 3) shared, 1 mine, 10, that was from before we met, he's also an adhd'er) I have suffered from adhd all of my life, from barely making it out of school to now managing and failing taking care of basic every day tasks.) I was finally diagnosed last year by a psychiatrist that tried treating my depression and anxiety before taking my adhd seriously. Even after expressing the concerns that I feel like those conditions are FROM my undiagnosed adhd, I still never got anywhere with him. I started with a new psychiatrist who has changed my outlook after one visit, I realized that my emotions are not regulated like others are (it's extremely bad) and that I have terrible impulse control which is expressed by spending money. I've realized after reading this book that that is my coping mechanism. The problem is that I'm just realizing this, I just had my aha moment after reading this book. I don't even know what it's a coping mechanism for though because it's something that was just a normal part of my life. It's a terrible issue because I know me and my boyfriend are in debt and he suffers so much with my adhd. He gets so frustrated with me, he offers support but at the same time tells me things like "you can do it, you just need to focus", which makes sense to him because it's easy to him. Me? I don't have the luxury of being able to focus. I just got hit with the realization that the thoughts in my head aren't the same as most other people. Other people don't have a nagging feeling to go spend money. It honestly makes me feel so stupid even admitting that. I am ashamed and want to follow the steps to restart our relationship so we can get past this but I need advice. He is starting the book tomorrow so I'm hoping that will help with his understanding of what I go through on a daily basis but I am trying to figure out this boundary thing.(and I feel stupid for even saying this too) She states in the book think back to a time when you were happy and what boundaries did you have set, well, there isn't a time. I've never had boundaries and I don't even understand what they are or how to implement them.. I am so confused with this. If any one has any tips on learning boundaries or information on breaking negative coping mechanisms and replacing them with healthy ones  or heck even what other coping mechanisms there are. That would be great too! I'm sorry for rambling. I'm about to take a deep dive into all things adult adhd but I would love to hear from someone who I can relate with on this journey. Thanks so much!