I'm sorry this is happening, and that it's so confusing (and I'm about to make it more confusing).
Check your test results - does it say "hsv2 isolated" or anything like that? Does it say "hsv isolated" and they are calling it type 2 because it's vaginal?
This is important, because your husband could have given you genital hsv1 if he has it orally and performs oral sex on you. It would then become ghsv1, but not all doctors know to order a type specific culture.
About half the adult population has oral hsv1 (think cold sores), but 90% won't ever get any symptoms, so they don't know they have it, and you can transmit it without any symptoms.
If they did type it, and it's for sure hsv2, yes, your husband could have had it for 25+ years and not known it, or you could have had it for that long and not known it. Many people have very mild symptoms, or no symptoms, and don't know they have it until they transmit it.
Make sure your husband gets a type specific IgG blood test. If it is negative now, but positive later, it's a new infection. It won't determine if he got it from you or someone else, though. You should also get a type specific IgG blood test. If it's positive now (assuming you just got this sore, and it hasn't been longer than a couple of weeks), and it's negative now, it's a new infection. If your culture was typed and it's hsv2, and the IgG is negative for hsv2, it's a new infection.
Transmission stats for men to women for hsv2 (assuming sex 2-3 times a week over the course of a year):
Only avoiding sex during an outbreak - 8-10%
Adding condoms OR daily suppression - 4-5%
Adding condoms AND daily suppression - 2-3%
So if he has it, and never got symptoms, it's quite possible that you wouldn't get it.
If he doesn't have it, and you do, it's also not surprising.
Ghsv2 transmission, female to male, over the course of a year, assuming sex 2-3 times a week:
Only avoiding sex during an outbreak - 4-5%
Adding condoms OR daily suppression - 2-3%
Adding condoms AND daily suppression - 1-2%
I know it's easy to suspect your husband of cheating, and I don't know if he did or not. If this is your only evidence that he did, it's not enough. If you've had other evidence, then he may have, and that's for you to determine. Medically speaking, either one of you could have had it for years and not known it.
You should read the Herpes Handbook at https://westoverheights.com/herpes/the-updated-herpes-handbook/ It's free, and written by one of the world's leading experts on herpes.
Yes, this is a lot of info at once. Take your time, let it process, and we're here if you have other questions.