I found myself in exactly the same situation. For a lawyer not to bring that to a client's attention when the original deal is being negotiated is an egregious failure of responsibility, IMHO. My original agreement was written for 5 years or something. As that date came up, I was all excited until I received a letter from my lawyer who advised that her lawyer was ready to sit down to discuss the next 5 years! (WTF???). I remember his exact reply when I asked when the payments ended and his exact answer was "When she says...".My biggest mistake was not having the lawyer ask for some sort of ending to the payments...
In my case, the previous 5 years saw my kids finish university or college (100% on my dime), she had taken on a new common-law partner, and I had remarried. She had a job (she had been a stay at home Mom) so a lot had changed. Oh ya, she had a paid-for house, and I had a mortgage. And the kids still living at home (which changed monthly!) lived with me, and had little, if any contact with her. I found the prospect of sending her money every month to sit on her ass mildly offensive to say the least. (By way of background, our marriage ended when she was discovered, by my daughter, to be having an affair!)
Fortunately, my ex and I had remained on pretty good terms, so we met for coffee and I just said I was done. She had soaked me for almost $1 million and I just said that that was all she was getting. I was prepared to put my business into bankruptcy and get a minimum wage job at Home Depot.
Thankfully, she agreed and eventually signed an agreement to that affect, very much against the advice of her lawyer.
I let my lawyer I wasn't happy with him, and he told me that the agreement we had was "normal", which it might have been, but it was still a huge oversight that he didn't bring it to my attention.