Im confused and feeling anxious...

UPDATE: Thank you so much for all your comments. I was in need of hear/read some other point of views and opinions, other than my relatives comments (they make it more difficult) I feel a little bit better and been thinking more in a positive way and analizing carefully. It is true about finding a man with no kids at my age...but other than that i really love this man and his kids are really nice. I will talk to him for sure about my concerns or thoughts.    
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Im having a hard time right now. I been dating this man for 6 months, and now we are engaged. He is a divorced man and has three young kids. He is the perfect  man i always wanted and i love him so much. Now that i said yes to him, a week later after he proposed i been having deep thoughts about my future with him. I never never thought about dating a man with kids 😰 but love hit me so hard when i met him, and we felt a strong connection with each other. We also said that it feels that we know each other from long time ago. I met his kids like a month later we start dating and they are nice kids and funny. He said they like me. But now i feel anxious thinking about my future with him and the kids, im not sure how to handle being a step mother and idk how the mother of his kids is going to be 😰(i dont know her at all)  I really dont want to feel stressed with drama. And i been thinking about having a kid or kids with him, he said yes and i do really want to have kids of my own, but im afraid that my kid will grow up in a family like that with half-siblings. I feel bad for saying yes to him when he asked me to be his girlfriend and now for saying yes to marry him. 
I dont know what to do. He is all i ever wanted to find in a man. Im afraid to make a decision of leaving him and regret later. Im already in my 30s and i dont want to be single and may be never find someone else or find someone not worth it. But at the same time, bcz i want to have a kid, im afraid that the older i get the difficult would be to have one. I love him so much there is no doubt about it. Its just me picturing myselft as a step mom and im scared not being able to be a good one and have a stressful life.