I have not dealt with severe depression, but I have dealt with other invisible illnesses that have no effective medical treatment, and I can understand the frustration at the inability of others to understand.

All I can really offer are the same empty platitudes you have doubtless heard before, and a suggestion to find something new to do. In all seriousness, I do suggest looking for a local game store that offers an open game night for Dungeons and Dragons or another role-playing game. It's a lot better than saying "just go out and meet people," at least.