I second what redcrx has said - the blokes guide to pregnancy is fantastic.
I am now 14 weeks pregnant and found the early weeks really tough, like you I was sleeping huge amounts ((getting in from work, kipping for 2hours, then waking up eating and going to bed from 8pm-6am). Although me and hubby talk a lot, I just didn't have the energy to explain to him how I was feeling proerly, I felt I was letting myself down, just by not doing silly things like getting the washing done when I should, letting the housework slide a bit, not plucking my eyebrows, not making the effort with myself or anyone else..... don't feel bad about it.
My hubby and I had a few wors, I think he felt left out more than anything, we'd found out I was pregnant, but instead of romantically sitting up til the small hours thinking of names and wondering what they might look like I was crashed out on the sofa and he was coming to bed on his own! This gets better trust me, I am now feeling more human (not super-human, admittedly, but human all the same) .
Anyway, not only was he laughing out loud whilst reading the book he also said it made him understand things a bit better and he thought it would be useful if I read it because I would understand how he was feeling too. (I haven't read it yet, maybe I am a bad wife :-P).
I'd suggest telling your fiancee exactly how you feel, you really need to pull together right now, but don't ever feel bad for feeling tired and not being your usual self - your body is working flipping hard at the moment! XX Keep posting on here, I am sure you will find it helps to talk to others going through the same thing xx