I found myself in your words. I'm 37 years old and I'm more mediocre than 17 years ago. Suicide is an option, but I'm mediocre also in this area.
Thankfully, you are mediocre in that area.
There must be something you enjoy doing, which if you work hard enough on, you can achieve some level that you can proud of. Within that area, you just might find some niche where your mediocrity is useful for others. As I've mentioned above I right mediocre software for the command-line (my niche is commandline and ncurses). Some people like what I do, some don't.
As far as suicide, besides the normal medical paths, please also try meditation seriously. There are enough threads in HN on that -- there are great secular meditations such as mindfulness or just breath watching -- so i won't elaborate.