So i told Serana to wait in Fort Dawnguard so i could go off to do some stuff and then come back and get her, but i cant seem to find here any where in Fort Dawnguard, any one know where i might find her or where she might have walken of too?

She's often just sitting against the wall in the main circular hall, surprisingly easy to miss. Apart from that, she might be having a look around elsewhere, maybe looking at a bookcase or something.

ill try and look around some more but i really hope that this dosnt ruin my progress forward

so i used Aura Whisper to see every one in fort dawguard and she's not there, could she have wandered somewhere

yea, sometimes when you part ways with her, she tells you that she will go to darkfall cave or the cove or any other place you visited in the mq