Need advise

Okay ladies. This is going to be kinda long but I really need opinions. There's a 13 year difference between me and my SO. He's 31, I'm 18. We've been ttc for three months now. He already has three kids, the oldest 7, and youngest two next month. I've only met the youngest child because he says he wants to prepare his kids, but we've been together for over 8 months now. I hate to admit but I do stalk the mother of his kids page! I'm scared I will catch him in something! I know it's something I shouldn't do, but her almost 10 years compared to my few months isn't anything. I've seen screenshots between them but I just am so nervous that he still has feelings for her. Do you think it's because of my age? Or do most women feel this way? I'm also having a battle with myself that I want my first child to be my parters..I just am lost. Please give me some advice!