How old is she? DD is 10. It's definitely not something that comes naturally to her, in part I think it's because she's quite smart, articulate etc, and so many things came so easily to her or just didn't matter that she finds it quite hard when she actually has to put effort in.
But things we've done that have helped her:
When she says she can't do something/ something is too hard/ too scary - just constant reassurance that actually, ANY progress is progress etc. eg I think she's got a fear of heights but can't articulate it. So things like monkey bars or some of the climbing frames she finds hideiously difficult. But we encourage her, stand by, and point out that yes, she did go one rung further than last time etc (because when they're in that spiral they can only see that they can't go the WHOLE way, not that they've made progress).
Things she's terrified/stressed by we talk about in advance and then very specifically about after. eg first school residential. Didn't want to go, was terrified etc etc. We had very specific conversations about how she feels that but we KNOW when she comes back it wil be okay. And then when she DID come back, we pointed it all out again (because, again, they get irrational so I know from experience if you try to be subtle they'll change the narrative in their head or promptly forget that they even found it hard and had to overcome it).
Sometimes we force her to do things she doesn't want to do - it's a thin line here so getting this right is tricky. But again, ti teaches her she CAN do it.
Offer her specific help to improve things and I am not hugely sympathetic to any braod "no, nothing will help" statements. So she was getting stressed about some stuff in maths she couldn't do. Obviously nothing will ever work, she will never crack it, she's useless etc. When we discussed a tutor she told me it was pointless..... needless to say, she's nailed it, she does also ahve a tutor and is doing very well but the thing I'm MOST pleased about is that last week she told me she was struggling with a particlar maths thing again without any of the hysterics that came last time, and she asked if I could let her tutor know so that they could work on it next time.
Also for botg DC we try to help them learn to put things in perspective. "Yes DS, maths is really hard for you but you're really good at these other things. Also, you're very very good at working at things that are hard, so can you apply those skills to improving your maths?" Or "Yes, DDOG is sick and she could die and that would be super super sad. But we'v ehad so many happy times with her and I will always be so glad we've had that - do you remember that time she jumped on DCAT's head and he was so cross? hahahahaha"