Wow Jim, you're being hard with him! He's not stupid. Just insecure.
Hello Charlie, welcome to this forum
Probably this James is really just a friend for her. But that doesn't matter. It's not about James, and not about Jessica, it's about you.
That she's the only thing that matters to you is a very bad situation. That makes you dependent on her. Girls feel that and don't like it
It's fine that she has other friends and that other things matter to her. That doesn't mean that you're not important to her. You should have other things that matter to you too. Don't you have any hobbies, friends, or other interests?
You should learn to trust her. I had a boyfriend once, I loved him very very much. But I left him, because he was very jealous. It's impossible to live in peace with someone who has doubts about your love or who is constantly afraid that you could love someone else. I can understand that she hates it when you say such things.
I guess the reason why you feel addicted to her is that you don't love yourself enough. What you don't get from yourself, you need her to give it to you. That's not healthy. The good news is that you can change that!
Don't judge yourself so much. You're not a baby or a whimp for feeling like you do. That's perfectly ok. You're just insecure inside. I think a good solution would be to learn how to love yourself more. Concentrate on yourself and on your life. Be nice and caring to yourself. Think good things about yourself, things that make you feel happy. Do things that make you feel proud of yourself. She'll admire you too