Your partner has hit middle age. They have been questioning everything about their lives, their very existence, what they are doing, where they are going. Sometimes these questions become too much, and they completely rebel from your partnership. Their midlife crisis destroys your life as well as theirs. In the end, it is too much and the midlife crisis causes your relationship to break down.
It is a tragic time, very hard to deal with, as many memories of happy times together will come flooding back to you time after time. The sad part is, that person you loved is gone, they have moved on and the hardest thing is, they have had more time to prepare for it, and when the announcement comes, it leaves you in a state of shock.
Your life is now seemingly in tatters, your hopes, your dreams, your plans all gone and wiped out. They are off having fun, doing what they "want", while you are left to try and cope and figure out what has gone wrong.
So what can you do?
Firstly, you need to realize that this is their crisis and not yours. It may feel like you are the one in crisis while they are seemingly enjoying themselves, but that couldn't be further than the truth.
Secondly, you should start looking to form your own life. If you can, appear nonchalant regarding their behavior and this in turn can work your way as they begin to realize the universe doesn't revolve around them after all.
Thirdly, seek counseling. Many people before have experienced what you are going through and there is countless help and advice out there for you. The sooner you deal with it, the better you are going to come out of it.
There are also countless publications out there with advice, information and help for you to understand what is happening and how you can deal with it.