You know it's funny that your sadness is on the brink of your daughters wedding. That is exactly when mine started about 4 years ago. I thought it was just all the excitement and confusion going on planning the wedding. But it wasnt, It was anxiety, depression, panic starting and It sure did a number on me too. I had to quit work, my whole world fell apart. I couldnt be happy for nothing. I finally went to a dr. who prescribed an antidepressent, it took awhile but it finally kicked in. I am now in therapy and that helps me alot too. As far as having nothing to show for your life? what about that daughter of your's you raised and is about to get married? That is very much something.
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Our wonderful health care is free to all but try to find what you need in a timely fashion. Most people are not diagnosed until too late with cancer and stuff cause as they say "there aren't enough doctors, there aren't enough beds in the hospitals". Neurologists take up to a year if not more to get into, ortho surgeons up to 6 months, most southern Ontario Canadians go to Michigan for bariatric surgery or heart attack issues becuase most would die if not treated sooner on our system. Yes it is free to everyone, but it is not available. You will still see the rich get at the front of a lot of lines for specialists so there is still palm greasing going on. I hate this buracracy and bull sht that goes on with our medical - this is why so many of us commit suicide. We truly have no help here.
This highlights one of the major problems with socialized medicine. If you live near the states you could easily find a mental health professinal who will see you on a weekly basis. This can be done even without a referal in many cases. If this isn't possible go to a member of the clergy they can often help you and may even have group meetings you can attend.
Also here in the states the county usually offers free or reduced cost mental health hep and group therapy for those who qualify. This may be possible in Canada as well.
Well it definitely sounds like you have depression. Whether or not it is a temporary problem I cant say. I can suggest staying active and meditation (buddhist meditation, clear your mind, dont focus on your problems).
Find a hobby to keep you active. Tell your partner how you are feeling. What do you enjoy doing? If you answer "nothing" you are just fooling yourself, there is something you enjoy doing. Find it and do it!