Maybe you should try some not-supposed-to-be-sweet herbal teas, e.g. rooibos, if you haven't already.

Or some real black, white, green, or oolong tea. For real teas, though, you should look up brewing instructions. Most people overcook their tea, oversteeping black tea and using boiling water for green, and it comes out bitter and disgusting. It was a revelation for me when I tried properly prepared tea.