So, I played as a Dawnguard, and had Serena turn me into a Vampire in order to access the Soul Cairn (I knew I could get cured later... and I also hit the wrong button........) Aaannnyways. Long story short, I promised Serena's mother I'd totally come back for her once her husband was dead. And hes dead, only now I cant seem to return to the soul cairn.... even though I was told I would be able to.

Help? D= I wanna save Serena's mom.

I also wanna go back and do the other quests that I didnt do while I was there...............

Did you cure yourself? If so, maybe that's the case.

Have you tried becoming a VL from Serana again?

Yes, I did cure myself in order to do the final attack on the vampires, cause the Dawnguard leader was all like "GTFO you monster" and wouldnt talk to me...so I thought I had to.

Yes, tried that too, the portal still zaps me and wont let me through.

Honestly though, you're not missing much.

Serana's mum also adds potions and a blood potion to her little collection of supplies you can help yourself to. But still if you cannot return it is no great loss. Getting Serana to soul trap my werewolf Dawnguard recruit Aussie and getting the soul essence back while in the Soul Cairn, created no problems for returning. Braggen went in as a Vampire Lord on the side of the vampires and had no problems returning. Maybe it is just a bug that if you change your form once you return you cannot re-enter the Soul Cairn. Is there an option for Serana to soul trap you so you can return, rather than turning you into a vampire?

This^^^^ The Soul Cairn may be re-entered at any time unless you chose to become a Vampire in order to enter, then cured yourself of Vampirism as you are no longer classified as undead.

http://www.uesp.net/wiki/Skyrim:Soul_Cairn