I recently did all the tundra express missions but when I wanted to go back there, I couldn't use the fast travel there, instead I had to go through Three Horns Valley. Is there anyway to fix this?
What I think you're describing is not being able to use the fast travel station at the happy pig motel in three horns divide. If that is the case, you'll need to complete the mission "No Vacancy" which you can pick up nearby on the echo attached to the bounty board there.
Completing that mission turns on the power to the three horns divide fast travel station.
If you mean you're using a fast travel station but tundra express is not on the list of locations, that's a different issue, so please let me know if that's the case.