Re (from Salamander
"I didn't know it was so important to you, but now I know, I won't do it again, I'm sorry, now I understand. You are important to me."
Either he is a really poor listener, or a really smooth liar. Good at damage control, familiar with what has worked to convince you to give him another chance in the past.
You did tell him it was "so" important to you, you warned him it would be a major dealbreaker. So either he did not hear, or he is pretending not to have heard. You'll have to take what you know of him and decide which is the more likely case.
Some people can forgive (x) amount of dishonesty. Can you forgive this much? Some can forgive (x) amount of carelessness. Can you forgive this much? He has been dishonest, or careless, or both.
By the way, if you ever get curious and peruse my big list of meds
, you may be discouraged by it but don't. One reason I provided the whole list is because every person's mind and chemistry is unique. So a med that works for one person won't work for another. Zyprexa may not be your answer; you may find that something else on my list is the more helpful to you. Or even that you need a med that's not (yet) on my list. But perhaps the list is something you can start with when seeing a pdoc, and my list of diagnoses is something you can start with when seeing a therapist. Things for them to look at. You might bring the lists to your appointments and ask the doctors what they think.
As for people who lie through their teeth, I have met at least a handful or three in my day and, like you, it just boggles my mind how they could do that. Maybe it's a habit that develops by degrees. They start out with "little white lies," get comfortable with that (and with its short-term rewards), then slide down the slipperey slope to bigger and bigger lies until finally they are lying so much that they don't even remember what the truth is anymore, and they're not even conscious of when they're lying anymore. A really good liar can even convince themselves, right?
You'll have the pain of this breakup to struggle through anew if someday you should ever meet a new poly partner. Trust issues are hard to overcome. But don't give up on polyamory altogether. There are success stories, where the people involved are honest, honorable, considerate, and constructively communicative. Those virtues are what make it possible for poly to work.
Glad you are feeling a bit less isolated during this difficult time.