My partner has 2 children from previous relationships. I want a baby and he isn't keen. Advice please!!?

My partner and I have been together for just over a year. He has 2 boys from previous relationships aged 10 and 5. The 5 year old lives with us as my partner has full custody of him. Shortly after we got together I told him that I would love to have a child of my own at some point and asked him whether he would want to have more children, and he said that he would when the time was right. I have been feeling really broody for a while and the other night said to my partner "how do you feel about us trying for a baby now?" He looked quite shocked and said he thought it was too soon and that the time wasn't right. I now feel a bit rejected and almost a bit resentful that I've been expected to play stepmother to another women's child yet not having my own needs met. I'm 31 and my partner is 37. He hasn't given a time limit, he just says 'when the time is right'. Since our chat, I've become even more broody. He says he has never loved anyone as much as me so I don't understand why he doesn't share my excitement about us starting a family together. I guess it's because he has already had 2 and it isn't the same for him but for me it will be the most amazing thing I ever do and feel crushed he doesn't feel the same.