I'm not that much of a newbie..... I'm stuck in the mortuary looking for a way to get through the portal to the fortress of endless unhappiness, or whatever it is, and I've found where the portal is thanks to my lil square buddy, but I can't work out how to make it open. Trias says go there with some regrets, and I figure - I've slaughtered my way through most of the known world(s) so far, surely there's enough regret there? (kidding). I've wandered around and around that preparation room god knows how many times, Hacked'n'slashed my way around the rest of the building, but all to no avail. Am I confused? Is the pope catholic? Any help would be gratefully received, I was damn near ecstatic to find myself nearly at the end of the game and now I'm stuck. Again.

You're probably in the wrong room and detected the wrong portal. The room you want is the original room where you woke up. Just go there and walk to the north of the room (to the fence after which there's a black pit; you know, the hole in the middle).
A dialogue window will pop-up and you'll be able to proceed to the Fortress of Regrets.

A tip: Take a lot of supplies with you. You'll need them.

Leave excess baggage before you enter...

That's the room I'm in, no dialogue though.... have lots of supplies anyway.

Move around Dela. Move around.

Between "bed" you wake up at the very start of the game and northern end of room that "bed" is in ("black hole" in the middle of the floor, as Tal said).