hangingin - I am going to go against the grain here.
My friend, who is 24 years older than me, (let's call her Dee), had been married 23 years, she was 44 and her husband was working away every week Monday to Friday (a great job opportunity in his field, for 1 year, to help them get on in life and they didn't want to relocate). He was a lovely, dedicated, family man who would do anything for his family. He wasn't one for going out with 'the boys', he loved spending his evenings & weekends with the family & friends. A lovely, sociable, friendly, caring helpful guy, who we all loved. Being away from Dee & the kids every week, all week, he got lonely without even realising it really and was pretty much an easy target for a woman he worked with. I'm not excusing him, nor completely blaming her at all, but he honestly didn't chase this woman, she did him and he just (very stupidly) didn't see it coming. They started just having the occasional dinner with people at work, then on their own, she asked him if he could help her fix a few things at her house etc and it gradually ended up with them having an affair. After a while, he told Dee about it. She was 'DISTRAUGHT', however, he was her life and she did not want to lose him. They'd been together since they were very young and had 3 kids (6, 11, 16). She told him how much she loved him and that she would forgive him, sadly he still had to go away every week as they needed his job to pay the bills (she worked as well, but they needed both incomes), he continued to see this woman for a while, Dee was (very understandably) in a right state and I told her not to be such a mug, to tell him to stay away, to tell him it was over, I told her that if he could be unfaithful their marriage wasn't worth saving etc (I was very young at the time) etc etc etc Much to my shame now, I was quite pushy and I could not understand what the fuck she was doing letting him come & go as he was. I was the only one she told, she was too embarsssed for their other friends to know - she felt a failure. She cried on my shoulder most nights for months, I ended up staying with her & the kids (M-F and weekends when he stayed with the OW (had to work of course ) and it was awful.
However, he came to him senses and stopped seeing the other woman. He still had to work there for a while - but eventually got another position in their local area.
Dee took a long time to get over this betrayal, but eventually she did (pretty much anyway, it never goes away 100%), but that was many years ago now, they love each other to bits. He has never strayed again. He knows what an utter bastard he was, he makes no excuses for his behaviour and he realises how lucky he was that she 'hung in there' while he was being a complete fuckwit.
There's always another way of looking at things & dealing with things ...
I have also been in a relationship where I was cheated on, I forgave him and shouldn't have - he did it again and that was that and I wouldn't put up with it once, not again.
You have been together 27 years (longer than a lot of the people on this board have been alive and certainly a lot longer than most have been adults for) which of course doesn't mean they don't give great advice 99/100 times and if you'd only been married 10 years I might be saying the same! However, I think that if in all that time this is the first time he has been like this (it seems most likely he is having an affair, but you still don't know that for sure and even if he is...), maybe you shouldn't throw the towel in too quickly. All I am saying is that you know your husband, you know what he's like, how he has been for over 27 years - don't be swayed by me or anyone else on an internet forum to do take a particular line of action.
I will be thinking of you and you have lots of support here - no matter how you decide to deal with it.