The biggest argument on here seems to be 'you can get stuff there' - which is true I guess, but I don't particularly want to.
Last year we were in Iceland for quite a short time, DD and I both were unwell - we carried on but had quite a packed itenerary and having to stop/ plan routes via the pharmacy, negotioate opening times etc is just a bit of a pain. Not insurmountable (and in fact I did have to do it in the end) but I'd rather just have the stuff there. If DD spikes a fever in the night, I'd rather have calpol to hand rather than having to find a 24 hour pharmacy.
I'd rather get the cheap as chips generic antihistamines over here and take a sheet from the box with me, since I know I'm going to get prickly heat and these help.
I'd rather buy the suncream that I know works for me, as well as the stuff that suits DH and DD, here at home as and when I see it on offer, and take it with me than have to go shopping for what's being sold at huge mark ups in the touristy shops if we're going on a beach resort type holiday. I like to put the once a day stuff on in the morning for DD and DH needs it as well as he'd burn as soon as you'd look at him otherwise. Still top up, but they both do best with this on first. We ran out on holiday last week and there was no once a day stuff to be found anywhere - DH burned with the other stuff we bought there, despite it being factor 50 and applying it liberally. DD was ok but she had the UK suit/ swim hat etc anyway.
Plus there's just the fact that I quite like holiday shopping and get weirdly excited about miniature anything including toiletries, so it's part of looking forward to going away for me.
I don't think wanting to go prepared/ pack more than a bar of soap for 2 weeks necessarily means you have travel anxiety.
I've never found it so overwhelming or impossible to work out what I need though, but perhaps there's more going on for OP and this is just one more thing and a bridge too far.