You cannot reset the nether without making a new world.
There always are nether fortresses in the nether so keep looking. I would install a mini-map mod so you can way point your portal so you can venture further without the worry of loosing your portal. Perhaps the morph mod or another mod so it can allow you to fly so you can travel faster.
Happy hunting :)
EDIT: just rememberedDo note that they can be affected by the Generate Structures
option, so make sure that's on (It's on by default.)