So I was in Darkfall Cave and I held the A button on Serana and it gave me the option to tell her to wait. I know I'm probably finding out late but I didn't know you could do it this way. So I told her to wait in some random spot just because, and after scavenging some bodies for my arrows I kept on moving to the spot where you get the water and move on to the next area. After I walked for a bit I noticed she wasn't with me and I thought she might have fallen back down or something. I walk all the way back to where I told her to wait and she's still standing there. There is no option to get her to follow me again and I can't tell her we should part ways because she tells me she can't leave, we have to finish the quest and all that. I've tried to fast travel, she doesn't come with me. I also tried to get someone else to follow me, but they say "looks like you already have someone following me". How do I get her to follow me again? I know I'm going to need her for the rest of the quest, will she just randomly pop up near me? I haven't tried it yet because I don't want to go all the way through the rest of the quest only to find out I could solve the problem right there.

I chose not to risk getting the bow only to discover I could not do anything about Serana, best to go back to a save before you asked her to wait. If she gets stuck or pretends to be a boat anchor she can magically teleport with you under some circumstances, but not it seems if you ask her to wait.

My last auto save was right after I asked her to wait and I don't have any other saves before now, so I'm screwed?

If you have autosave on why do you not have the autosave from the area before?