So yea, I dismissed Serana, and she said that she would meet me at darkfall cave. She obviously lied, and I found her in the castle. But, I can no longer ask her to follow me anymore. Can this be fixed? I cannot reload, I have leveled up too much to revert to a previous save, and I deleted most of them.

Your not alone dude. I posted that glitch in the Bug thread over in the Hardware / Software section a couple of days ago.

Well deleting your save files was a foolish thing to do, you cannot be that desperate for space, and you need to always keep a save from before you start a quest or ask Serana to leave or wait, since she is probably, more glitchy than most things in the game. Do you have the bow you went to Darkfall cave to get?

If not you can try turning the console off and reloading the save
Fast travel to the cave and see if she follows
Talk to her about the bow and see if the option comes up
Just go get the bow yourself, but if you need Serana with you to start the next part of the questline you are probably doomed.
In which case you load what ever your latest save was before this, start a new character to do Dawnguard, forget about doing Dawnguard or just sit and hope Bethesda get around to fixing the problem eventually.

Or you could reinstall Dawnguard if you don't mind doing the content over. I'm wondering how this flys over people's heads. I saw a friend who had over 200 hours make a new save file and delete the old before trying a reinstall. Your characters not doomed yet.