I'm not a lady, but sometimes I am a gentleman.
It's not about trusting a new female friend. It is about trusting yourself, trusting in your own judgement. The only reason you will question someone else's honesty is if you've been snookered.
This lawyer that is trying to represent you both, and is copying your confidential legal emails to your wife, this is a breach of trust. But you at some point decided you could trust them, and now you find your instincts were wrong.
You sound like you trusted your wife and believed in your marriage, and your instincts were wrong there. So now you are beating yourself up because you got fooled and as you look around you, it isn't all the people you see that you can't trust anymore. What you can't trust is your own feelings and senses and instincts.
I've been there too. Here is the reality, we aren't perfect. You will go through life and make mistakes and miss seeing things, sometimes things that are obvious to everyone else. You will have faith, because faith is a natural human need, but you find that your faith is often misplaced. You will go through life and have the rug pulled out from under you. It will happen, time and again.
The defence is not to never trust, because if you choose that then you are damaging your life in countless other ways. You must trust and give people a chance to show you what they are made of. In some cases, that means giving them enough rope to hang themselves. That is still trust.
The defence is to keep a part of your life and your heart for yourself. Have your own time, your own friends, your own activities, your own savings and your own space. This isn't just a physical protection, it is an emotional one, because you aren't so completely invested in these people you trust. You have time and space to yourself so you can shake your head and see them clearly. You can have fresh perspective. And if things go wrong, there are parts of your life that are unaffected and you can retreat to those and bounce back that much quicker and easier.