Thank you for bringing your question to Just Answer!
I can see you are in a lot of pain. It can be so hard to be so in love with someone you can't be with all the time.
One thing you can try on the days that you can't be with her, or don't hear from her is --in order to keep yourself from going 'crazy' with worrying and wondering, is to write her a letter that day. Start it in the morning and keep adding to it as the day goes on. When the time comes that you would usually talk to her (but can't, for some reason that day) write down what you would have liked to say to her.
It almost doesn't matter if you ever show her the letters or not. But it will be a way for you to get your thoughts out on paper, which should help you clear your mind so you can get through the day without too much pain, as it will feel as if you really have been talking to her throughout the day.
This will help in the short term. The previous expert has already addressed the long-term issues you face. But until then, keeping a running letter to her that you add to during the day will help reduce the obsessing feeling.
And in answer to your question of finding out if you are losing her...only she can answer that for you. You're in a tough spot, because she also has to consider her children's lives in her decision. If her husband finds out about the affair, he could try to take the children away from her in a divorce. Even though she may love you, she may not be able to make the decision to put her family at risk. If she gradually decreases her contact with you, it may be her way of pulling back, but not wanting to hurt you.
Be careful with your heart...whether she stays with the family, or leaves her husband for you, there will be a rocky road ahead. Even if she leaves him for you, there will be a court and custody battle that will create a lot of drama in your first years together. It can be hard for a relationship to survive that...
If you have insurance, it might be worth having a therapist to talk to to support you through this turmoil. Here's a link to help you find one, if you think it might help to have a neutral third party to talk to .
I wish you all the best,