I can't seem to find the cave that I go through to get to the Dawnguard fortress. My waypoint is, legitimately, on a mountainside. I'm standing as close as I can get to it without being a goat and climbing it. Any help here? What's the name of the cave? Anything, lol?!

On your map the waypoint is on the mountain side, on your compass it takes you somewhere else. That somewhere else is the cave.

It is close to where the Stendarr have there hall. You will see a path.

Go to the new waypoint then Turn off all other quests n the waypoint should lead u right to it.