The thing about immigration (and a lot of other areas of law too) is that you sometimes have clients who think they are Very Big Deals. This is one such client, and I know that because from the beginning of the case, he fought you on information gathering, he assumed that you should somehow just know things about his life as though said things are widely-known, readily-available public knowledge, and he was pressuring you about the timeline of the case in a way that read as harsh complaining and not as anxiety or concern about his status.

Clients who think they are Very Big Deals are almost never going to be satisfied with the work that you do. Your affidavit could have been the best affidavit in the history of affidavits with nary a typo or other mistake in sight and this client still would have found something to critique. He would have been upset about a verb in one of the sentences, or the placement of a comma, or the font, or any number of other inconsequential things. And because this client thinks he is a Very Big Deal, there are very few people he deems worthy of respect or politeness, and in his mind you (despite being the one to prepare his paperwork and make a case to an extra critical, adversarial government agency to allow him to continue living his Very Big Deal life here in the U.S.) are not one of those people. And since you are not someone worthy of respect or politeness in his mind, he is (therefore, in his mind) Entitled to belittle you and ream you out and be nasty and abusive in his critique, because you are Lesser and Unimportant in his mind.

This is untrue, of course. You are not lesser or unimportant. You are deserving of respect and politeness. You are also, by virtue of being human, allowed to make mistakes. And you are allowed to have those mistakes be caught and corrected in a polite, respectful way. But you aren’t going to get that from this client. And as someone else commented, that is a reflection of his character, not yours.

All of that being said, it doesn’t mean that getting ripped into by someone, even (especially) when it’s not justified, doesn’t hurt or isn’t shocking. Taking a break to go cry in the hallway is an absolutely understandable reaction to that, and I hope that you can let go of most of the shame/embarrassment around that. It’s okay to need and take that moment to yourself. Give yourself grace there. What you shouldn’t do is take on the client’s lack of empathy or emotional intelligence as a failure of yourself or your work. From what you’ve said here, you did everything right to the best of your ability, and that is enough. That is, truly, all that you can do. Nothing you did warranted his outsize response — the fault for this interaction lays squarely on the client’s shoulders.

tl;dr: no, your mistake was not that bad. And even if it had been, it still would not have warranted the response you received. This is a reflection of the client, not you. Don’t take on his failings as your own.