This is huge. I agree completely with addressing this very directly after thinking on what you want to say for awhile lest you (OP) accidentally blurt out something terrible again during the difficult conversation. I say this because what's written in this post isn't quite enough--it's an explanation but it's easy to accidentally say the wrong thing here. It's easy to accidentally slip into, "I was trying to be honest but I should have been nicer" which is not a good idea.

Here's a silly (and overly verbose, sorry!) anecdotal story for comparison that is not nearly as bad as what you said to your girlfriend. I mentioned a post from this subreddit yesterday about the guy who was obsessed with his gf looking hot (and being a huge dick about it). I was telling my partner how much I appreciate that he doesn't mind me when I'm lounging and a little sloppy at home; in fact, I know he thinks it's cute. I know it's his preference over all. He agreed and said how he prefers it in a way that came across as very sweet and then unnecessarily blurted out that it's actually his preference over all and always has been, not just with me.

It was not a bad or unreasonable thing to say at all. I know this will sound really petty but it stung badly because that's not who I am in most areas of my life. At home, I like getting into lounge wear and being comfy but due to a lot of childhood issues, I absolutely will not leave home without being put together, and I'm happy with this. I don't dress fancy unless there's a reason but I'm not ultra casual, either. Being super casual is absolutely not who I am anywhere but at home. I have some pretty serious self esteem issues that were directly caused by my partner (over a year and a half ago and he's been wonderful and supportive ever since) that actually tangentially relate to this issue.

So while he's apologized and made amends for the past, is supportive and tries to build my self esteem, that statement hurt. He tried to correct it and said he misspoke and I believe that he didn't mean to say it that way but I do know that's his taste preference (which again is fine in and of itself but it's a very sensitive topic). My minor setback in feeling attractive is lingering today and might linger for awhile because of our overall history. I don't think my boyfriend is a jerk, said anything inherently bad, or meant any harm--in fact I knew from the beginning he completely intended to be sweet and compliment me. He meant to let me know how much he really does like it when I'm lounging around. He really did mean to boost my self esteem but it backfired which isn't entirely his fault.

What you said was way too blunt. Like, no shit she's not literally the single most beautiful woman ever to have existed. She's incredibly beautiful to you. She really needs that validation from you and not only did you deny it, you compared her negatively to other people which is really, really bad (and part of my history with my own partner). I don't mean to be an asshole and drill this into your head. I know you know it's bad--you're dealing with the fallout. But you really need to delve into this with her and understand that even after you do, her feelings very well might linger for awhile and that's okay.