I bought a new light fixture to replace an old outdated light-bar the bathroom of my apartment. The building was constructed in 1926 so everything is super old. When I took the old fixture down, I noticed it didn't have a ground wire. Just black wires and one white wire. The white wire was grouped together with other black wires and there was one single copper black wire on its own. The two clumps of wires were connected to the old fixture but I can't remember where the single black wire was... I have a photo but I'm not sure how to attach it.
Sounds like you have your switch leg ( switch loop ) mixed up. You need to isolate it. It is the wire going to the switch. A simple continuity test will determine which wire is the switch leg. You need a continuity tester and most voltage testers check continuity. As for the ground, there is none available unless you provide a new circuit to the panel assuming it is grounded correctly. Since there is no ground you can remove the fixture ground to avoid any potential problems and to save space.
Here is an example drawing I made (Revision 2):