I didn't read all the post replies... I read a few up top....
Is it time to call it quits?
No way.
A few things..... as long as you are still alive and have your mental facility in tact for the most part, you can write music. Look at the people who have no arms who paint beautiful art by holding a paintbrush in their toes or their teeth. It might take longer but they still express their art.
In another forum ...Band in a Box.... there is a member there who is disabled and in a wheelchair and apparently has limited use of hands, and can't speak anymore. Yet he still composed music using the BB software and posts in the user showcase from time to time....and is active in the rest of the forum. Your description of yourself isn't quite to his level of disability, so based on what he's doing in music, you don't have a good excuse.
There's always a way....
So...another short story. I attended a songwriter's convention in LA a few years back and got to hear the great songwriter Jeffry Steel as well as several other hit writing songwriters including Jason Blume. Steel spoke about his songwriting and said that he writes every day even when he feels like he doesn't want to. In other words, he forces himself to sit down and put the proverbial pen to the paper. Others above have mentioned working with a collaborator. There are a number of songwriter sites where people are looking for musicians to put music to their lyrics or vice versa. Some of the better songs I have written and been a party too have been collabs where someone sent me some words on a piece of paper...or via e-mail. It's amazing sometimes to see how simply reading the words someone else has written can very often, ignite the spark of creativity in yourself.
A bunch of my songs are co-writes with Pat Bishop. I also have a number of co-writes with some other writers, but pat and I have done probably a dozen songs or so. We met through a collab with a 3rd person many years back. We're worlds apart politically and philosophically, but in the same groove when we're writing a song, and that's all that matters. She's a lyric writer and sends me her lyric ideas. About 1 in 5 or so turn into a finished song. It's magic when you realize that you're witnessing and involved in that special moment at the birth of a new song and the creative juices start to flow and the song starts to take shape from the simple written words of another person.
So there you have it.... find a way to get back involved. As long as you're waking up every morning, you can be writing and sharing music and being a part of the creative process.