How do you return to the Soul Cairn after you have finished the first quest but want to go back. I tried turning back into a Vampire but that didn't work either.
Now when I try I just lose health.

Can't seem to get back in. I just stand at the bottom of the stairs going nowhere.

I only returned after completing the main quest so not sure if you can before or not but i had no trouble on my second visit, apparently you become attuned to the soul drain and it has little or no effect, try eating the soul husks.

If you were a Vampire when you first entered then I believe you need to be a Vampire to re-enter. If you are a Vampire try exiting Valerica's study and then returning before attempting to enter the Soul Cairn.

Edit: awesome avatar, OP.

If you ask Valerica about entering the Soul Cairn when you're not a vampire she'll tell you that after the first time you have a connection to the Soul Cairn and will not take damage.

That option is bugged for me - I couldn't do the quest "Chasing Echoes" because firstly I couldn't put all the ingredients into the cistern, and then I couldn't enter the Soul Cairn, so had to keep doing consol command to get through the stage. When I finished the main quest and wanted to get Valerica to return, again I found I couldn't get back.

I have managed to finish the DLC, and will uninstall and reinstall to see if it helps.