I'm not a very sentimental person either, but I would suggest thinking of a lot of casual compliments. Most people like to be complimented. Just slip them in throughout the next few weeks. I would advise compliments on her abilities, on things she thinks she is good at and things she doesn't think she is good at. If at some point you need help with something, go to S instead of someone else and then thank her and tell her how amazing she is. 

Additionally, I don't know if you can, but maybe offer to get her something from the concert? I've never been to a concert but I think they sell shirts and stuff?  And does she know she is your best friend? You should let her know, if not. This happened to me a long time ago, I didn't know I was someone's best friend until she moved away and told me later when she made new friends.

To be honest, Leafpool, I think the best thing you can possibly do is to simply keep being her friend. Keep supporting her and make sure she knows that you love her and think she's amazing no matter what role she gets, no matter how many contests you enter together. Your friendship is not built around competition, it's because of the fact that you both like writing stories, listening to Lindsey Sterling, and a million other things.

But I have to ask: Do you feel guilty because she's jealous of you, or is it just the fact that you are getting things she isn't? If you think she's jealous you could try gently bringing up the topic and mentioning how much you wanted her to come with you to the concert and how you wish she could have won, too. Be sure to back off if it seems like it's upsetting her. Trust your own judgement about whether she'd rather talk about how she's feeling or not. You know her best!

If you don't think she's actually jealous, just try arranging a sleepover or something so you can hang out together. It'll show her that you think she's amazing no matter what and you want to hang out with her, even if you can't do it at a concert. Best of luck with everything, Leafpool!

Thanks for all the great advice, people! 

@GreenMango, Impunity Jane--She does know that she's my best friend, and as neither of us have many other friends anyway it's fine. We've known each other since we were really young and we're both homeschooled, so we don't have huge friend groups or anything. And our three other friends who know each other and us all know that S and I are really good friends, so they're not offended by the whole "best friend" thing.

@Cockleburr--wow, this is a lot of help! Thanks! I think it's mostly just that I feel guilty that I'm getting all these things and she's not. I don't think she's jealous...but who am I to say?

@GreenMango again--yes, I am going to offer to get her a Lindsey Stirling t-shirt. 

One option is to ask/talk to her about this directly and come up with something together, the two of you. Do stuff with her to make up for the time you spend away. You could also take some time to shift the spotlight and celebrate her. Throw a random party in her honor if that's your kind of thing. Give her a special sort of gift. I don't know. Don't go so overboard that you miss the point of it. Awkward apologies might be in order. Talking about it is usually a good idea; tell her how you feel and such, then let what happens next come. Hope this helps!