Stop torturing yourself by reading the texts, they won't give you the answers you want. Sadly neither will your DF, cheaters generally don't really know why they cheated unless they have been through therapy. You can drive yourself insane trying to understand cheating. It's probably nothing you have done or could have prevented, it will be all on her, we can only respond to situations like these we can't stop them even if you could see it coming.
Even if you're not sure if you want to split. Start protecting your finances and assets, if you only have a joint bank then set up your own individual account for your income and savings and only pay whatever you need to the joint account to cover bills while you live together. If you have a gold rolex given to you by your great grandad, passport etc get it in a safe place out of the house, your DF is not going to react as you expect from now on. People who are hurt or have been caught out act in unpredictable ways she isn't the person you know. She certainly isn't the woman you fell in love with, she has just shown you that.
Get advice about how to split the mortgage/assets etc. again even if you don't know you want to split yet, it will help you prepare if you do. Citizens Advice can be good here, as can a local solicitors. Your DF need never know but you will need to know your rights and responsibilities. Do it quickly and quietly.
Take some time away from your DF to have a think, even if it is a walk inthe park or drive in the car/train etc. You need time to breathe and get your head straight to think through what you want to do next. Being in the same space as her and not getting answers won't help you. Try not to get drawn into arguments, keep things calm when you are around each other, even if she kicks off, walk away to a safe space.
Be prepared for anger, tears and begging, yours and hers. You will have a huge roller coaster to ride emotionally for a few weeks. There are quite a number of really helpful threads here with plenty of advice and ask the folks here, we all have our own opinions but in general you will find support and advice here.
But definitely plan to leave and split assets, you don't have to go through with it but getting knowledge on these things are really helpful. If you need to move out of the house quickly, do you have somewhere to go?
Time out of the relationship when cheating has occured can be really helpful to get yourself together again, however make sure you retain access to the house and any assets you own if you are the one doing the moving. You can also ask her to leave but if she is on the mortgage she doesn't necessarily have to.
I hope you can speak to your DF when things settle down a bit so you can sort things out amicably but people do change their minds and can lash out. Keep that in mind and everything in writing.
If you choose to try again, therapy can help but keep in mind it doesn't often work, the trust has gone and that is a big thing that is hard to get over. Remember you don't cheat because you've had some stress in your life, loads of couples go through meh times and stress and manage to stay faithful. She cheated because she wanted to and she felt she could hide it from you.