@KillerRabbit, firstly I do have to ask where you take it from that Mystra is painted as a crazy ex and especially that she is made out to be one by Gale himself. I have read this assumption in many fan theories, but not from Gale himself. I admit, that I only went through his dialogue with a lawful/good "such ambition is horrible" angle once, so I am not sure if it changes anything. In my experiences with his dialogue, he is even protective of Mystra and stands up for her when you make snide remarks about her in Act 3 and he isn't on the war path. The most negative thing I recall ist that he feels a little salty that she asked him to die, which I cannot fault him for.
For me the hubris reading - also in connection to the story you related - misses the plot twist in Act 3. Hear me out:
Gale is a wizard, he gets his power from study and using his intellect to solve problems. While he is generally very cordial to clerics and bards, he's competitive with other wizards and somewhat dismissive of sorcerers who just got talent. He also points out how important study is at every opportunity, a lot of his banter resolves around this, especially with Lae'zel who seems to be a fellow nerd.
So, during the first act of Gale's folly, when his girlfriend shows him what magic is possible and beyond his reach, he gets a spot of imposter syndrome and, instead of being inspired to study harder, asks her for a leg up. She says no, he pleads and pouts (framed as the wrong thing to do by Gale himself) and eventually sets out to get her a very over the top present, a lost part of herself. This of course backfires and in Act 1 Gale believes that he was punished for trying to push to hard. This differs from the story you told about Ged, because Gale never tried to get himself a powerful spell from a forbidden book, he tried to get a present for Mystra to sort of guilt-trip her into letting him in on some of her secrets. His assumption that he got punished for his transgression is also wrong. It does show that he only blames himself for everything though, leading to depression as @SteelTempest pointed out above.
In act 3 Gale learns that the orb was something completely different from what he thought it was and that Mystra did not punish him at all. She just severed their connection, because he suddenly became very dangerous to her and - this is my interpretation - she didn't quite know what his intentions towards her were. I say this because she instantly reaches out to Gale if he shows good will in the Shadow-cursed Land, and her dialogue during the audience to me also sounds like she trying to suss out where he's standing. After the audience, Gale is furious with himself that he made such a rookie mistake and - at least on the forgiveness route - is mostly furious with himself for having been so stupid. Gale's "facing his shadow" moment is meeting Mystra because really everything hinges on this conversation. If he tries to ascend without having spoken to her, he will explode because he wasn't "armed with the truth".
The irony is, that in his desire to become an even more powerful wizard, he ignored what makes a good wizard in the first place: Research and study, two things he holds dear. The moral of the tale isn't that power or even ambition is bad, but that throwing your ideals in the wind to get somewhere quickly leads to ruin - even God-Gale needs to have learned this lesson in order to succeed.
In regard to your last paragraph and again with what @SteelTempest wrote in mind, I don't like to think of love as one-way-street. Yes, Gale offers to sacrifice himself so neither his lover, his friends nor the world come to harm, but his desire to do so is very much fuelled by his low opinion of himself. I am not sure if you ever got the conversation with him at the brainstem, during which he tries to persuade you into letting him explode, but he is clearly not in a good head-space during it. So, while this conversation is painful, I love that it indirectly forces you to tell Gale that he quite literally means the world to you because you'd rather risk everything than loose him.
While Gale's "the exploration and acceptance of the self and the other" from Act 2 mostly means sex in the context he gives it in, I really like it as a general cornerstone for love - or any form of relationship really.
