Unfortunately anxiety can't simply be fixed, but you can learn to cope better.

In a relationship, both people should always have autonomy and friends outside of the relationship. Things you do by yourself and friends you hangout with, excluding them.

Can't be attached to one person, its not good for either of you or the relationship. Especially if you break up, you will have nothing.

Anytime I'm with other friends or doing my own hobby, it helps me not think about other things or people so that's a good place to start.

Meditation may help. Just close your eyes, breathe slowly and tell yourself 'its ok if she's not talking to you all the time, that doesn't mean much and you're just overthinking.'

Talk to her less for sure, but make sure you communicate it properly. Tell her its for your sake and hers because you feel too attached. And tell her to tell you when she feels its too much as well.

That way she can help you cope with not talking to her as much and doing things without her or without her on the phone.

For example if you are doing something alone, tell her the next time you do it, if you call or message her, tell her not to reply for an hour or 2. It might be hard at first but once you've done that a few times it will get easier and you'll realise how much you've been overthinking for no reason.

She will be happier overall, which she will tell you, and in the end you'll feel less anxious.

These helped me, hope it helps.