Planned outing with friends - now feeling sad and lost

Personally, I wouldn't bother to include her in future. The risk of any event clashing with her camping trips would be a sticking point I'm afraid.
Does this friend ever invite you along to anything she has planned?
Lockdown has made many people more sensitive but it has also made many people selfish, complacent and self centred, you don't need people like that in your life.
Enjoy the exhibition and let friend A plan her own life in future.

I’ve found over the years that it’s best to only include one friend in an outing. 3 makes for an awkward dynamic. I once tried to organise outings with two really good friends thinking that, if I liked them they would like each other, but they really didn’t get on!

MayBee70 yes, I made that mistake. I learnt from it.

No you're not being the spoilt child, Friend A is.
You chose an outing you knew she'd enjoy, presumably she agreed to go and that you would get the tickets?
Has she paid you for her ticket?
For her to even suggest that if the date later clashed with another outing she'd miss yours was just rude.

I hate to think of her being rude because she's a lovely person and generally very kind but she can be a bit thoughtless sometimes without meaning to.
I don't want to not include her in future because it would hurt and upset her unnecessarily and I'd feel I was being vengeful and spiteful. I couldn't do that to someone who means a lot to me. I was just hurt and taken aback that she seemed to be dismissing our outing so easily without thought to my feelings.
Yes, to be fair, she has paid for it.
She was not only happy for me to get the tickets, we'd been talking about going for a while and she seemed really happy that I'd taken on the job of booking.
Her life is busier and more social than mine, but her plans don't include her husband (long story). Because she's now so busy, I don't get to see her that much now. Maybe I'm a teensy bit envious (yes, I know I shouldn't be, but I'm basically rather insecure and though I try hard not to be, it's something I struggle with). I don't ever want that to influence our friendship because that is my problem.

13 years ago I had a devastating split with a long term close friendship for which I accept much of the responsibility. Mentally I was going downhill for a while because the friendship was becoming toxic for me for reasons I won't go into (and I'm certain for her too) but I was really trying to keep my own insecurities hidden and not allow them to affect this friend. Unfortunately my worsening depression and anxiety contributed to me losing that battle. Following the break, I was unwell for a few years and had therapy which made me feel a lot worse. Finally I began to feel a lot better, with the right meds and time passing, family support and that of the few loyal friends I still had and cherished. It had all coincided with serious family problems, an assault on my daughter and a subsequent court case. Things seem to come all at once sometimes, and though I struggled on with it all at the time, I think I wasn't up to the task of riding the storm!
Anyway - that's just sketchy background, and though I'm far, far stronger and happier now in some ways, I still get flashbacks of former dark times and it affects how I see things. I also find it hard to know whether I'm being unreasonable or not in feeling as I do.

Thank you all for your insight and thoughts.

You are allowed to feel how you feel xx
It is quite normal to feel sad an rejected even if it is unreasonable. In this case though I think you have done something thoughtful and she has forgotten or taken it for granted, I'm not sure she knowingly hurt your feelings either.
Be kind to yourself
I's have loved to have come along

it's just that she has a much fuller, busier, and easier life.
she probably has no notion that you would be hurt by her candidly stated preference.
she is motivated by the event. whereas for you it is more about your self perception, and emotional issues.
this makes you more vulnerable, to the natural vagaries of people's preferences and impulses.
so how you feel or react is just a fact.
i don't think feeling are reasonable or not.
but it might help to try to rationalise a bit about it all.
actually i think it is quite brave of you to try to socialise, with all these hazards that affect you more than others.
all the best.

Maybe she said it flippantly knowing that she hadn’t double booked the date? Rowantree, I think you undervalue yourself. I’d love to have such a kind thoughtful friend as you. I like to think that your friends think the same, too x

I'm with you on this one. If I've made arrangements then barring emergencies I stick with them.
I had a friend 'once' who was fond of doing that sort of thing. She invited us then put us off twice as her cousin was coming. She suggested another time but that was it. She had form, she and another couple of work colleagues along with myself, had arranged a night out. She rang the very evening and told us someone else had invited her out so she dropped us for them....nice.