Stay out of the crowd >> You don't have to be in the middle or front of the crowd to have a good time, no matter what anyone says. You will most definitely still be able to hear the act, and the huge majority of stages will have screens for you to see the performance, even if you can't see the act on the stage itself. Hanging towards the back means that you have more room to breathe, and you will most likely be surrounded by more like-minded people.
Go with someone you trust >> I can't stress enough how important it is to feel as comfortable as possible in a place that stresses you out. I went to T in the Park with a best friend of mine who 100% understood that I hate being in the centre of crowds, and she made sure to check in with me regularly to make sure that I was okay. It's great to be with someone who will say "yes absolutely" when you ask to move somewhere else and lead you away from a place where you feel uncomfortable, because it will ultimately make the experience so much better for you.
Wear something comfortable >> Again, with that whole comfort thing. I find that when I feel uncomfortable in something I'm wearing it's all I can think about, and so something you feel cute or sexy or just plain comfy in is a good shout. It probably sounds like it wouldn't help with anxiety, but you need the least amount of things for your overactive brain to think about! And, as a side note - if you feel uncomfortable wearing skirts and dresses in crowds (which we shouldn't, but is sadly the world we live in), but do want to wear them, invest in some small shorts or hotpants to wear underneath. It genuinely makes me feel more protected, so again there is less to worry about.
Go for the day >> The idea of camping at a festival actually terrifies me a little, because I know there's no real escape from the crowds or *gulp* mud. Let's be real, you need comfort when you're anxious (have I stressed that enough in this post already?) and so the knowledge that when the show is over you can head home to your safe place can be quite calming. And, don't be scared to say that you want to leave. If you are having a bad time, it is absolutely okay to remove yourself from the stressful situation.
Look after yourself >> Be organised, make sure you are eating and drinking enough (I always lose my appetite when I'm stressed so I like to set reminders to fuel myself!), and check in with yourself. Are you breathing okay? Do you have a headache? Do you need to sit down for a little while? Or find somewhere a little more quiet? Also make sure to communicate with the person you are with.
Have fun >> Festivals are meant for having fun and enjoying good music. Soak up the atmosphere and listen to your favourite songs! Sure, check in with yourself to make sure you're ok (and whoever you're with) but the ultimate goal is to have a good time. Take silly photos and sing along badly, you're allowed.
This turned out a lot longer than I was expecting! I'm sure there are so many other things I could mention as well - feel free to leave any other tips below!