Pregnancy and how it affects the mother is an intense stew of physical and hormonal changes. Each child may come with a different "stew" recipe. Sounds like the father of her original child didn't stick around, so maybe there's some sub-conscious anxiety as this brings back memories and feelings that preceded that event/issue, as well. In any case, how she's feeling now may or may not have any carry over to how she feels once the child is born, so don't put too much weight into that.
You can't make her more cheerful or peppy about it. If she's struggling a bit more with the emotional side, all you can do is be there to offer whatever support she needs, and quite often that will mean doing nothing but listening or even being the receptacle for her fears, outbursts or frustration. Don't try to "fix."
This is a good introduction for you to parenting. If you look at how you are framing your question, you are excited, and you want your experience be even better by her joining you, emotionally.
Parenting is nothing, if not sacrificing your wants and even your viewpoint/perspective that focuses on you, for that of your child and family. To be a good father, you will need to learn to set aside your own desires for others, whether that be the unpleasant task of enforcing unpopular rules, giving up activities and social interactions to be a glorified transportation service, going to parent meetings for activities and organizations and, yourself, playing nice with other parents who may be maddeningly annoying.
Start off by being entirely supportive of her needs during this period. The experience will be life-changing and positive, even if it isn't all laughs and giggles now.