Oh dear OP. This is the worst feeling. I'll just tell you my story (shortened version of what could have been a film script).
I was a happily married woman, two beautiful kids - one an IVF baby, achieved 2nd go. Very much wanted by both parents.
Managed to conceive 2nd child naturally (only natural pregnancy I had ever had), not long after 1st child was 5 months old. So two babies.
Very happy family - making plans for future and refurbing a huge new home
Knew each other from being 16 - no money, no guarantees of future £
Loved my inlaws, all GP's got along, all lived locally, in same community.
Financially very comfortable, no worries or arguments over £
He'd worked very hard building up his business and I was a SAHM and worked for our family company.
Then...
ExH seemingly had a mid life crisis. I had no idea at the time, other than him seeming to prefer going out with friends rather than me - one or two on the periphery who dabbled in coke/ecstasy/whatever other drugs they thought made them cool in their 40's 🙄
Started wearing bucket hats like Liam Gallagher 🤣
Never put his phone down (before the days when adults as a rule never put their phones down)
He jumped like a jackinthebox once, when I surprised him one day in the garden whilst on his said phone - weird. Said I gave him a 'fright'.
He started 'nitpicking' at me over things/causing arguments he was never bothered about before; spending money, sex (called me a frigid bitch once). I cried buckets.
I caught him (only the once) looking at himself (seriously) in the large mirror in our hallway - he used to take pride in 'never caring what he looked like' - was a family joke.
He started 'jogging', going out until 11pm after work (he always worked late but that was nothing new.
He flirted massively with one of his friends' girlfriends (very attractive, and slightly younger than us). People commented to me about this. Still; nothing really for me to get overly suspicious over. Too tired with kids to be paranoid.
Went away for a family weekend for xmas, and on the drive back he screamed and swore at me for asking him to slow down - we had two babies in the back of the car. No reason for him to be speeding.
Reason for above behaviour
He'd been having an affair. It only came out after I stole his phone when he was asleep and read the messages (this was before biometric passcodes). He then tortured me by being unwilling to tell me who with. I found out after two days of begging and pleading - and yes I knew her. I knew I would. Utterly distraught. He then told me he thought he 'loved two people'. That was it for me. Out he went, even though house was in his name (we had other properties between us) and I immediately secured a remortgage on the family home. (Yes he tried to remortgage very soon afterwards). He then became what can only be described as evil, towards me, and my family. There were other family members brought into this through no fault of their own (business ties etc..)
Red flags
Flirty, funny, engaging, reasonably good looking, confident. (I'm not an insecure person nor a jealous one, so these traits didn't bother me).
I appeared to get genital herpes when we were in our 20's from nowhere.
Very short temper and very aggressive when drunk. Loved a drink, but not during the week.
Conclusion
It took nearly 3 years and it to escalate to Crown Court his behaviour was so atrocious.
He had another affair with the girl I originally thought he was dallying around with (his friends girlfriend), by this time they were married so he broke them up. He then returned to the original mistress, who he seems to have remained with, amongst other known affairs. Somebody messaged me anonymously from France, telling me he was having an affair with a school parent... I've no idea who that one was.
I found various things after he left, one of which was a video of him having sex with an 'unknown' woman, in Amsterdam, after a stag do.
And a 'secret' bank account where he bought lots of sex toys.
Like you, I was blindsided. I had known him for over 20 years. It happens, and better you've found out now, than later on. I have since happily remarried, ended up doing a job I loved (retrained) and have lived my best life. I even managed to co-parent with dignity, something I really struggled with, but my kids have grown up to be very well balanced, successful adults. I have tried hard to keep the ugly bits of their dad's behaviour private, and to all intents and purposes, I'm so glad I did. I have to rely on the odd 'your dad provides very well for you' (he does), kind of positivities, rather than 'your dad is such a lovely man/fun/briliant at x,y,z/everyone loves him' kind of thing, that I was lucky enough to hear about my dad.
The truth is, their dad is a sociopath, a narcissistic cunt, and is severely lacking in emotional intelligence. I'm not perfect, but I"m not the 'nutter' he made me out to be, and never was. I also wasn't the frigid bitch he mentioned either - my second husband can vouch for that 😄
You will survive. Wishing you every bit of luck; please don't go through with the wedding. XX