I can only offer moral support (sorry I'm not very good at it) and encouragement and obvious suggestions.
If you've been on medication 20 year then you must have had depression for even longer. Your strength and will to fight are amazing. With that much experience you don't need me to tell you that sometimes you just have to keep slogging on until things brighten a little. I hope you'll reconsider seeing your psychiatrist. That and your inner strength seem to be your best resources until something else comes along.
Can you talk a little about DBT and how it helped you?