Hi emre43. I am new to this site. Your post caught my eye as I am going through very similar circumstances. I am roughly same age and have never worked due to many issues. I really feel for you. I think that not being able to work because of illness is like an illness in itself, it can really bring you down further. May I ask, do you use the Do-it.org website service at all? I have found it to be quite helpful in uncovering new volunteer opportunities. I have to say that the reply you got was pretty rude considering you were only offering to help and for free!
It might be an idea for you to draft a general email with which to send out to every future volunteer job you apply for. For example, introducing yourself and simply saying that you are looking to lend your time to help others, benefit community ect and mention that you are a caring person with an interest in animals, nature, art -(whatever you like) and then just give your contact details and say that you hope to hear from them in the future. This is what I did. It really saves time having one fairly short, general email and for me it takes away the over emotion that I get when I get into the "I MUST get a job NOW, i'm a failure! frenzy" which sees me emailing anyone about any job at 3am! This way, you are just a kind stranger with time on your hands who wants to help. Who wouldn't want that. Please persevere with the volunteer work, when you get there it will be worth it I'm sure.
I actually started volunteer work today, it was quite a bit overwhelming at first but I got through the day to my surprise! What I have learned from today which is worth mentioning, is to take the distance to travel to/from the volunteer work into account before you apply. My work is over an hour on the bus which means a very early start for me and I get travel sick, also it costs more. All I'm saying is that when you get accepted for the right job and you begin the work it will be a great feeling, probably with nerves attached and try to do whatever you can to ease yourself in Best of luck with the volunteer work and please feel free to chat anytime. And by the way I really admire you for trying!