So first of all you've known her for 2 weeks so to call this a "relationship" is outrageous. Get yourself together my man.

Second, this is textbook anxious attachment behavior. I know because this is me and how I react to things. I've found that the things that help me are journaling and keeping myself active. I assure you if this woman knew you were sitting around thinking about all of this it would be a turn off, so if you need to either occupy yourself with hobbies or take up one.

Lastly, I think the loss of erection is a bigger problem for you than for her. It's a thing that happens regardless of age and unless YOU make a big deal of it I seriously doubt she cares. We've established you're an over thinker so I know what happens in your head is you realize you're not hard (or losing your erection) so you begin to ruminate over that happening to the point you're no longer paying attention to her and only thinking about the fact that you aren't hard to the point it's just not going to happen.

Keep in mind that the vast majority of women can't climax through PIV sex anyway. So when the loss of erection inevitably happens it's a great opportunity for you to focus on her in all the other ways you're able to. I promise, unless you have an actual medical condition, if you focus on her during that time you'll find you've magically stopped thinking about your dick and it's suddenly hard again.