Hi,
You got excellent answers from everyone above. One thumb to each. A woman does not just change her mind after 15 years of marriage. Let me tell you the possible causes of her change of heart: This is coming from a woman who had been in this situation.
1. She felt neglected by you all these time either physically, sexually or financially.
2. She is discontented in your sexual relations.
3. You don't show her much attention anymore maybe because both of you are very busy and preoccupied with your respective jobs, and responsibilities at home.
4. She has met someone else who makes her feel loved, important and valued by giving her a lot of praise, attention, and gifts that she value.
5. She does not feel attracted to you anymore. Maybe you have gained too much weight and this in turn affects your lovemaking.
6. You probably hurt her feelings without knowing that you have done so. Or perhaps you abused her physically or emotionally.
7. Perhaps she thinks you have another woman or you have been busy watching pornography.
8. She may think that she'll have a better life with someone else.
These are just possible reasons why she changed her mind about your marriage. I'm not accusing you of these things, don't misunderstand me.
What needs to be done is to have a heart to heart talk with her and ask her what the reasons are and if both of you can work this out. You may need to see a marriage counselor. If after these suggestions she still insists on separation and possibly divorce, by all means let her go and let her find her own happiness. Everyone is entitled to have a happy life. Hopefully she is not making a big mistake.
If she insists on divorce eventually, try to work it out between the 2 of you. Be fair in the division of your assets if you have a lot of assets. Going to a lawyer is very expensive. Divorce lawyers are very greedy people. They are like voltures ready to swallow you alive and take your money. Sorry to say this about divorce lawyers but that's the truth. Ask divorced people. I'm sure you'll hear a lot of bad stories about them. They are a Necessary Evil.....
Good luck and I hope that you'll straighten out this problem once and for all. Be strong.