He is the bad guy because he has told her this while on a much looked forward to holiday - at least by the OP - and not only was this while they are on holiday, it was the first evening of their holiday!
I strongly suspect that he is depressed about turning 40, and that for almost his whole adult life he has been with the same woman, and is now thinking that although he loves her very much (she is almost certainly his confidant, and probably his best friend to), he can't help wondering what his life would be like, and almost certainly what sex would be like with someone else. I am sure that their sex life has gone through being fantastic, exciting, comfortable, maybe occassionally a little boring, but mainly more comfortable than exciting these days.
Yes, all of that is sort of understandable, probably especially for a man, but his timing is crap and it is very shitty of him to do this while they are away, and presumably the OP is away, from any support that she hopefully has from loved ones back home.
@Swils1009, I got married at what is considered now to be a very young age, to a man just a few years older than me. We had children, then after about a decade he told me that he was leaving me for someone else. I was devastated, I still loved him so much, but I had sometimes wondered over the years whether he would ever regret marrying his first girlfriend - obviously, in the end he did regret it, although I hope he has never regretted having our children - who are all adults now. Even though I had children, so I came as a package.
When I eventually felt up to dating again - and having sex with another person (other than my dear ex husband) for the first time in my life - I actually had a brilliant time dating other adults, and experiencing life as a single adult for the first time. Within 4 years I was married again (to my 5th sexual partner), and we are still together now, and are both retired pensioners.
So please don't think that you have done anything wrong OP, or that you are no longer attractive, or that something - other than being together for a long time - was not good about your sex life, I truly think that if there is not another woman already lined up, that it is his age, and that you were very likely each others first serious relationships, that has had him brooding for a while now. Unfortunately, I am not hopeful that your partner can get over any feelings he has of "missing out" in some way, without him leaving and living a new life. He may agree to go to couples counselling with you, and that may even work - in a way - for a while, but I am not sure that he wouldn't still be wondering sometimes "what if...", and eventually he would probably leave anyway.
Please don't put yourself through all of that extended heartbreak OP, I really believe that if you let go of him now, you will be so much happier in the long run. I think that sometime in the future your partner may well look back and realise just how lucky he was to have had you, and kick himself for being such a stupid idiot; but by then (it could be 5, 10, 20 years before he realises it, and just in case you think about waiting for him, it may never actually happen) you will almost certainly be deeply in love with someone else, someone who can really appreciate you as well as love you, because they had already had the experiences that our young adult lives usually give us, that help us grow and mature into well rounded adults who know what they both want and need.
One last thing OP, you will get through this - yes it will hurt so much at first, but sadly life often does hurt very much when we have major life events happening, especially if we didn't want them, but sometimes even when we did - you will learn a lot about yourself, and about other people, and you will ultimately be in a much better and happier place. Ok, one very last thing OP, please don't decide to rent somewhere to live for a while, as it is extremely hard to find a landlord who will accept you having a dog living with you. So, even if you have to buy somewhere very small to start off with, please do so, as you say you want to keep your dog. I know the latter through a very difficult personal family experience - we did keep our wonderful four legged member of the family, we would have lived in our car, or a tent if we had had to!
I will be thinking of you OP 🩷 xx