Hello mummyjen. I have felt a mixture of feelings throughout my partner's pregnancy: absolute joy for us both, sadness for me, insane jealousy of her. It was always going to be me who carried our first baby, but I failed to conceive and so she 'had a go' to take the pressure off, and got pregnant first time. I was delighted and very upset at the same time: After years of waiting to try and then months of trying I didn't want to still be waiting. And of course I then made everything much more complicated by adding feelings of guilt etc. Our baby is due next week, and I am still jealous of her for being the one with the bump, being about to go through labour (yes, really...) and am still impatient to have a go myself. I think the trick is to recognise and be honest about these feelings, and in our case we were sure that these feelings were separate from my happiness: They should not necessarily be interpreted as meaning that you are not happy with the fact that you and your partner are having a baby. Congratulations by the way!
Separate from my thoughts and feelings about my own lack of pregnancy I have found it a wonderful and priviliged experience to observe another woman's pregnancy so intimately: Most partners of pregnant women are not able to identify with having a woman's body and are not able to anticipate the day that they themselves can go through what she is going through. My partner is also looking forward to being able to compare her experiences with mine when I am pregnant.
So yes, I think I know exactly what you're talking about, and am still waiting impatiently to be pregnant myself. I think you can be both happy for your partner and the both of you (er, all 3 of you now, or 4, 5..?!) and yet still feel sad, jealous etc without it detracting from your happiness.
How pregnant is your partner? Did you use known or anon donor? What kind of relationship are you hoping for with your donor - will you use the same one? How many questions can I ask?! It would be nice to talk to you.