TORN!!!!!😫😭😢

I have dating my bf for 5 months. He was upfront with me since the beginning he did let me know he had a baby son which was conceived before we starting dating. His son is now 10 months and every week he goes over the child’s mothers house 4 days out of the week to see the baby. I really feel uncomfortable with this I didn’t know that this was going to be the arrangement. I Love him and I absolutely want to be understanding because he is being a good father to his child! I also don’t want to be stupid either. Anytime I bring it up there’s a argument! I really don’t know what to do or how to approach the situation correctly!😢😢