Stressed and confused

So last year around this time I found out that my boyfriend, of 3 years and the man who raised my son from 4 months old, had been cheating on me and he had gotten some chick pregnant. Before this I was on birth control but we both were planning on trying for a little girl together. We had everything planned out for a year and a half basically just waiting on good timing. They ended up keeping the baby and it was in fact a girl his first biological child. We are still together but I cant even hold the baby without feeling jealous not of the baby its self just the fact that she's not MY daughter. I ended up getting off birth control while the girl was pregnant and now we are actively trying. But I just cant help but think am I wrong for the way I feel about the baby?