Friend forgot our plans

She managed to remember she'd booked a plane ticket & is going abroad then?

I'm really upset. I spent so long organising it. Am I overreacting and should I just let it go or say something
Why do you think you are over-reacting?
Why would you 'let it go'?
Has she even apologised, or asked if you are out of pocket for the things you have booked?

If she is neurodiverse then it would help if you were told about it...

And we all have to find ways to manage our weaknesses.

To be fair, if I'd arranged this with someone in the summer, I'd be in contact with them sooner than one week to go to confirm arrangements.
It was a lovely thought OP, can you get refunds on what you've booked or go with someone else ?

I'm neurodiverse, didn't know myself for a long time and my friends aren't aware because I don't really feel the need to label myself now but one of the challenges I have is that I'm incredibly forgetful and disorganised, i also get really anxious even before things i genuinely want to do with people i genuinely want to see. All my energy goes into keeping on top of things in my job so I'm not failing at that and by the time I come home I do not have the same mental energy to invest into other areas of my life. I'm fully aware that makes my flakey and I'm prone to things like forgetting to put things in my calendar and double booking myself. The one thing I hate the most about it is when people assume it's because I don't give a shit about other people. I care so much, when I make a mistake I will beat myself up over it for days but it's something I find very hard to change. If you know she is like this I'd say speak to her about it, explain the effect it has on you and give her a chance to explain her behaviour she might have a good reason that you're not aware of like mental health or adhd. Maybe she is genuinely just flakey and isn't considerate but if you call her out on it it firstly shows you care enough to be honest and secondly gives her a chance to either change her behaviour or explain to you what's going on.

I’d probably just pull back from the friendship a bit. She might be flakey and forgetful but in this instance I think she just doesn’t put a high value on your plans. I certainly wouldn’t be rescheduling a big day out for her.

I imagine she remembers what she wants to go to and doesn't double book those things...
If she didn't care enough about your friendship to write down commitments that is a pretty clear message.

Or, accept that friend doesn't care enough to diarise plans, move on and find friends who treat her better?

OP's friend is flaky when she chooses to be. I don't suppose she forgets to diarise work commitments, holidays and other events that she considers important

You are right to feel upset but, you know your friend is like that so regular reminders are useful. Set up a whats app titled birthday weekend in November and send regular notes ie make sure you pack your swimming costume, I've booked our, I'm really looking forward to celebrating your birthday etc. it just keeps things alive for her so she can't excuse she is forgetting.

I’m afraid she doesn’t value your friendship as highly as you do.
We had a friend like this. The last straw was when we and another couple were sitting at home waiting for them to turn up for dinner. Eventually rang and guess what? She’d forgotten. Kindly said that they could come round anyway, wouldn’t be long.

As the last time we went to them for dinner she had also ‘forgotten’ we were coming, I took the hint.

( last time I heard from her, she rang and asked me if I would like to canvas for her in the local election. When I had stopped laughing, I politely declined).

I’ve voted yabu, as you know she’s flaky. More fool you for booking events for a ‘friend’ who’s prone to letting you down and leaving you hanging!
sorry that’s not what you wanted to hear, but it’s the truth.