They way people's brains work when they speak is strange sometimes. Maybe talk to him about how you feel and why you feel that way. To me it sounds like he was well meaning and did not intend to hurt you. I've definitely said things to my partner and vice versa and neither of us intended to cause the other any pain. If he did however intend to be mean/hurt you I would suggest leaving the relationship or rethinking the whole fiance thing. But that's just me, I'd hate to be in a relationship where my partner intentionally tried to hurt me emotionally.

This could go so many ways depending on the tone he put it in. Like was he joking, did you really have a head injury, or was he just being an asshole and calling you stupid?

Sounds like a compliment to me. "You're so great right now I wish I'd known you when it was even better" I'm actually failing to find any way of taking it badly.

I agree that there is more than one way to take the comment. OP did you have a head injury or have a neurological condition that your fiancé is aware of? If yes, while it was a rude thing to say it might not be have been intended to be mean spirited thing to say. My wife, whom I love, has a neurological condition from a stroke in her early 20’s that affects her memory. She has habits and accommodations she makes to correct for this (like a very well filled out calendar). But it sometimes affects us when she has a memory lapse and accuses me of things I did not do or say. But this is fairly rare and I knew what I was getting into when we were dating. Sometimes I might wish she did not have this neurological condition, but I accept her as she is. Most of the time I do not call it to her attention, occasionally I do and then regret it. On the other hand if OP does not have a neurological condition then this was either a put down or a “joke” that fell flat. OP don’t make this a make or break issue. Judge the relationship based on the whole package. Sometimes people say things they regret or just plain shouldn’t have said. If this is the only problem you have in the relationship then let it go.

I'm impressed by all of the level headed responses. Good community here.

As a person who suffered two traumatic brain injuries (I received an unsolicited asskicking that put me in the news, and two years later a bad car accident by an insurance fraud scammer), I wish I met my current wife before then. My memory is all messed up and it changed my personality. I honestly think it’s harder on others than myself since I don’t even recognize the effects it causes (I have severe memory issues when I used to have photographic memory that lasted years for perfect detail recall), but this was a bit heartless what he said. Yeah, definitely talk to him about it. No matter if you have the tbi, or support someone with a tbi, it’s hard on all involved.