S.D.
I don't know how you feel and I can't tell you how you should be feeling. But, I can address a couple of your reservations.
First, 34 isn't old. It's not even considered "advanced maternal age" by the most conservative doctors, so don't let that affect your choice.
Second, do you always have issues with depression or just postpartum depression? If it's PPD, the best way to avoid PPD is to dry and encapsulate the placenta and take a pill each day. It works better than any drug on the market. Nature knows what it's doing when animals eat their placenta:)
Third, there's nothing wrong with raising your daughter as an only child. Even if you had a second, they would be about 6 years apart. They would end up being like only children anyhow. They are unlikely to ever be interested in the same things since their peers would be so separated. The average family has less than 2 children which means that TONS of kids are only children to balance all the families that have 2+, right? So don't feel bad if you don't want more children.
Fourth, if you're open to the possibility of more children, but dread going through pregnancy with your other ails, you can always look at surrogacy, adoption, and fostering.
Good luck with your decision:)