There are a lot of bitter people on MN Edna, who can only assume that because they have come across scumbags that other men who are not with their children's mothers are also scumbags. I would just ignore them. I think I am some what like you and prone to overthinking. This is how I am interpreting your posts, I may be well off the mark. You are 24 and you have fallen in love with a man with 2 very young children, and that is not what you expected in life. No one has daydreams about falling in love with a man with children already. Now you are 11 months in, and you can see how much he adores his children and you are unsure if he will a) really want anymore children with you in 5+ years time b) that he will find it all old hat. And perhaps you want to know now in order to save yourself heartache. I am afraid no one is going to be able to tell you the answer, and all I can say is go with your instincts, you know your DP best not us.
A lot of people are advising you to just relax and not worry about it an wait and see, but in 5 years time you could've wholly commited yourself to this man and his children at the expense of having children of your own, or you do have children with him but their daddy may not really love or want them, and is just doing it to please you? So how can you jsut relax 5 years is a long time to wait. Yes this is the sort of weird overthinking I can relate to, and I have to say sometimes not without merit, but more often than not apt to ruin your life.
Your DP has tried to reassure you deep down maybe you don't believe him? Or you are feeling insecure about something in your relationship? I don't know, you are still young and I would think that if this is bothering you so much now maybe your DP is not the man for you? Or perhaps to put it another way you are not the lady for your DP. Perhaps if it is bothering you so much walking away now may save your sanity. Being a mother is not easy bieng a mother and a stepmother is even harder, add in a whole host of worries justified or not, then that is not a pleasant life to make for you, your DP his kids and any possible kids you may have.
When my DH and started on our relationship I told him straight away, I want children, I want a family and I don't want fall in love with someone who might not want to go through it all again. My DH really wanted to have more children, he missed his son and his stepchildren and longed to have a house full of kids again. His face just beamed tbh when I brought it up, he didn't have to say yes I want to have more children it was written over his face.
I had a slight worries and fears when I was pregnant, hormones go everywhere. However I was so relieved in the early days of DD life that DH knew what he was doing.