My baby's health comes first during pregnancy but ob says to focus on me. Why?

Both true. This is typical for pregnancies in older women; they value their pregnancies and fetuses even more than their own lives - however the obstetrician's primary concern is for the mother's health (not least because healthy mother generally means good pregnancy outcome).
Healthy Pregnancy. I don't know what your doctor specifically told you but it is also possible that you may have misinterpreted the advice. An obstetrician's role is to make sure that both mother and baby's health is equally optimal at any given point. If you take care of your body, nature often assists you in having a healthy pregnancy. Wishing you all the best in your pregnancy.