So, boyfriend's super suspicious IG story aside, why didn't the RN test the sores on your lips? Does a dermatologist do the fillers? The derm could have done that, very easily.
There are lots of things that cause sores in the mouth, and and a flat white sore sounds more like a canker sore and not a cold sore. Lots of people think canker sores are herpes - apparently, some nurses might, too.
The thing about hsv1 is that many people get it when they're young, from well-meaning adults who kiss them and don't know that the sores on their mouths are from a virus or that they are infectious. About 50% of adults in the US have hsv1 orally, and 2/3 of adults globally do.
It would be hard to prove he didn't have it before and just transmitted it to you now, if this is what you have, or that you even got it from him, and didn't have it before. You could get a type specific IgG blood test, and if you test negative now, and positive later, it means it's a new infection, but people test positive in as little as two weeks, so the longer the wait, the harder it would be to prove.
Also, the hsv1 IgG misses 30% of infections, so if either of you test negative, it doesn't actually mean you're negative, making it even harder to prove anything.
As to your boyfriend, people with nothing to hide, hide nothing. I don't know if he cheated or was just doing something really stupid, but I totally understand your reaction.
Great advice all around from Auntijessie. And here is some other good news.... there are steps/changes you can take to help increase the odds you won't have another outbreak or that they won't be as long or severe (like taking the Rx you've been given, diet changes, healthy living changes, etc.) and other good news... most people tend to have less and less outbreaks over time.....there are advances being made in medicine all the time, clinical trials underway for new therapies, support groups for when you need to touch base, and so much more.
None of this is to take away from your situation and what I'm sure feels like a bum-deal, I know I've been there. I was devastated when I was told I was HSV-1 positive. Hopefully, after you've had time to process everything a bit more, you can also take stock of the positives in your life too? There is a whole world of people out there who have lived what you are going through, are still living it, support each other, and who are getting on with life. I wish the best for you.