Welcome to JustAnswer! I am a Licensed Professional Counselor. Please note, this is not therapy, but advice. I do hope I can help you though. I am so sorry you are being hurt this way, My heart goes out to you and your children, and your wife for that matter. I do believe there is hope though. There is always hope!
You may be very well correct. She may be going through a mid-life crisis. For whatever reason, she has become discontent in her current state, including her relationship with you. She is seemingly searching for happiness.
I often tell people who are going through such difficulties that if whatever they are doing isn't working, try the total opposite. I know this may sound cliché, but it has been proven successful numerous times. It is worth a try.
If you have been trying to change her mind, quit arguing with her. Don't plead with her any more. Give her the space she desires. I'm not guaranteeing this will work, but it is pretty evident what you are doing isn't working either. If you chase or push her you are liable to chase or push her even further away.
Please understand, I am not telling you to give up. I am only suggesting a change of tactics. Launch this by letting her know that you do care for her and that there is nothing you would like more than to have her back. Inform her you are giving her the space she seems to desire, but that is simply because you care and that you hope she will see what she has before it is too late. You them back off and give her that space you promised.
I hope this is helpful. I know it is difficult to swallow, but definitely worth a try. Let me know if you have any questions. I do want to be informative and helpful to you. Thank you!