If you have a save right before asking her to join you in the Mindflayer Colorny, use that.
If your only save is after you have already recruited her, try exiting Moonrise Towers (walk outside, pass the bridge, to the Moonrise Towers waypoint). You should be able to go to camp. When at camp, speak to Jaheira and see if she can stay behind. If she can, add in a fourth companion and try defeating Ketheric without her. Then speak to her in the Throne Room.
Not sure if that will work, but it doesn't hurt to try. It sounded like you really wanted to get Jaheira. I know it sucks losing hours, but I would rather redo the battle and get the character, than completely lose out on her after trying hard to keep her. Jaheira is mandatory in Act 3 if
you want to recruit Minsc and Boo.
