How do you support a child, whilst also hoping that they're prepared for disappointment?

[munemema]

Or is this my issue?

DS has a dream, he's worked hard to put himself in a position were it's a possibility but there's still a long way to go, both in terms of his own performance and things completely outside his or anyone else's control. He's in a good position and still working towards it, but there are still plenty of hoops to jump through iyswim.

It's great to see him so focused and committed but I am really worried about what happens to him if it doesn't work out. I don't know how he'll pick himself up or what other direction his life could take.

I'd like him to be prepared for disappointment but I don't want to give the message that I don't think he can do it.

He's 18yo, so not really a child but he is my child

Oh bless you OP.

I know it’s hard seeing them get their hopes up for something that might not happen.
Perhaps just drop into conversation something about “hoping for the best but preparing for the worst” and say “I’m sure with all your hard work you’ve got this, but if it doesn’t work out for reasons beyond your control, what would you like to do instead?”

It’s always wise to have a backup, a plan B. Sometimes it can even give you the confidence to chase your dreams.

DD did an Art degree, wants to make a living painting, but I encouraged her to do some work with children and now she knows she’d be perfectly content as an Art teacher and painting in her spare time if it came to it.

This is a real toughy and one we have dealt with, albeit that my ds is younger. He wanted to go to a performance related school but needed a funded place which are very hard to come by. He worked incredibly hard, got through to the final round but ultimately it was a no.
We always tried to hope for the best but expect the worst to manage the expectation. We have also tried to emphasise to him that whilst his actual "skill" is specific the characteristics which have got him to this point aren't ie. discipline, resilience, hard work etc. Also that even though this particular door has closed there are plenty more opportunities related to this so he should persevere to see if it takes him anywhere.
DS was gutted and is still very disappointed but he has got over it and we have found new challenges for him.

Look at some growth mindset stuff on the internet. literature or YouTube. It was primarily based on characteristics of successful people through their dogged resilience in the face of failure and challenges. How your approach to challenge and failure can, in the long run , help you out perform those natural talent .

Emphasise transferable skills. No skill, talent or experience is ever wasted.
So he works towards his dream and you mention how great it is that if he ever needs to xyz, then he's got the background to do it!