Anxiety is a problem that everyone has to deal with from time to time, but some people suffer from it way more frequently. It can affect people to varying degrees, often in the form of a person who would describe themselves as a worrywart, someone who's fretting about stuff all the time.
But just because a girl worries all the time doesn't mean she's not worth dating. You just need to be aware of some of these things first.
1. Telling her to stop worrying just isn't going to cut it. Sometimes it just slips out when you think you're saying something helpful, but a person who is worrisome can't simply stop.
2. Sometimes she'll just need you to be there and listen. Even if you can't solve any of her problems, even if she's you keep offering the same comfort over and over again, that's okay. She just needs you to be there.
3. She knows that often her anxiety is irrational. Anxious people know better than most that their worrying and freaking out is pointless in the long run. That doesn't make it any easier to stop, so you don't need to tell her that.
4. She might get overwhelmed and cry. Frequently. It doesn't mean you did anything wrong, so don't take it personally.
5. She's just as frustrated about it as you are. Probably more. Patience is key. If you show that you're exasperated with her, it'll just make her even more anxious.
6. She has probably worried about every aspect of your relationship at length. Worrisome people are great at getting inside their own head, to a fault, actually. Most likely they've thought about every possible twist and turn your lives might take and worried over them each in turn.
7. She will appreciate your compassion more than she can even say. Being there for her will take some patience and empathy on your part, but she'll pay it back tenfold and love you that much more for it.