I agree with people who say that it is probably the shared connection with her late mother in law that is at the heart of it. Maybe you have unintentionally said things that made her feel that you didn't understand.
I think you need to look for other interests and let your daughter come to you as much as possible. Don't invest too much emotion in the relationship as it sounds as if it will help you. It is very sad but there is no easy solution and you must protect your own mental health. See your doctor and don't resist if he suggests medication any more than you would if offered it for any other illness.
Can’t stop dwelling my problems - obsessive thoughts
I'm now seriously missing out - feeling miffed
I hate the name they're giving her - have to bite my tongue