Unfortunately I've been in your shoes (discovering an affair) and it's very hard but some things I've learnt is: (and also to balance out the leave him crew)
You as a person will survive it even when you think the pain is too much
There is more lies and eventually the truth will come out. He needs to completely and honestly disclose everything if you have a hope of survival.
Don't make decisions when you're in shock and be very careful who you tell. Some people just want the gossip and never hear from them again. Most advice is not to make decisions about your marriage for a year afterwards. Whilst it's hard it's true. (I'm 18 months down the line)
Don't talk to the other woman. They are liars with shitty self esteem (would you settle for being the other woman?) (obviously presuming she knew he was married but given feb texts probably). No good comes of contact with her. I did I ended up more hurt.
Set some clear boundaries about contact with her and him. He needs to respect them and understand the consequences if he doesn't. You decide what they are.
Make an exit plan even if you never use it it's very helpful in setting boundaries.
He didn't have a one night stand. Or if he did there has been someone else. Most likely is he's been seeing the same person since before Feb and now he's ended it finally she's escalated.
I highly recommend affair recovery videos on you tube for him and you. Go ask suzie website is good too. Letters to my husbands affair also.
Get a professional involved. Both individual and couples therapy for you both. The NHS can provide both in most areas with a wait.
We have a time out policy. If a conversation is becoming too much for either of us one calls time out, we separate for 30 mins, then come back when calmer.
We don't fight around our son and have discussions out of the house whilst possible.
My husband saved our marriage. It took time and there were circumstances which were different but the things he did were:
Became completely transparent. I have access to everything including his work email and apps (it was a work affair)
He answered every question I asked with no limits.
He cut contact with her to nearly nothing (they still work together as he's under contract for another 2 years)
He got therapy himself which has been life changing for him
We started couples therapy after a year.
He watched a lot of the affair recovery videos and digested their messages at length.
He became more compassionate and clear
We set boundaries and he sticks to them religiously
He will discuss things as an when I want
We did a lot of exploration about our relationship
My marriage is stronger than it's ever been but it has taken us both to put some work in. We are a partnership and whilst he did the damage he's done the majority of the work (I remained ambivalent about the marriage for around 9 months). We've learnt a lot about ourselves and our dynamic and it's far healthier. He's far healthier.
I am happy to answer questions but not judgements. :)