Have they been replaced by anything? In my case, I just feel completely blah about everything. All the future days start to look like the past, with no end in sight.

I think you need to solve this problem, a lack of joy in life. unfortunately I think others advice in this domain is often useless, however I will provide some anyway.

our brains are composed of different parts, and the lower levels determine our happiness, so finding joy is a lot like training a puppy, but the puppy resides within.

I read a book which was excellent on this topic, I think it was titled The Elephant or something. anyway, I would try an approach of behavioral cognitive psychology along with finding some joy providers.

joy providers include the right friends or some young pets or kids to work with. I know there are others for other people, but unabashed joy is infectious. good luck. walking the valley of death is no fun, but there are those who will help you through.