Hi, welcome to SI, the best club no one ever wants to join.
Gently, more than likely you don't have the truth. Cheaters lie and minimize and blame and manipulate...all of them.
Who is this other woman? A friend, a co-worker? Is she married? How can you be sure it ended?
There's a way to recover messages, I'd do a little research or call your phone company to see if they can help.
In the meantime, forgiveness is earned. I agree with another poster, you are rugsweeping like this never happened. What consequences has your husband had for his betrayal? Transparency, IC, Accountability for his whereabouts, access to his phone, emails and social media? Read books?
What is he doing to prove he will be a safe and faithful partner?
Why are you afraid of him leaving? He destroyed your marriage, you didn't do anything. He needs to own his sh*t.
When I discovered my husband's A, I asked hundreds of questions, repeatedly, some for years because a few things didn't add up. MOST of the truth eventually came out but it was because I was relentless and did lots of digging myself.
He cheated because he wanted to, not because there was anything wrong with you or your marriage. You deserve to know the root of why and where and how. Every question swirling through your head needs to be answered by him without defensiveness.