Screwed up all my permalinks

Hi,

For reference my site is here: https://lennyantonelli.ie

Stupidly, and without really knowing what I was doing, I decided to see if I could create custom URLs for posts on my site, as previously they were just numbers and looked a bit silly.

So for the first post, ‘Midde Ground’, I entered ‘middleground’ in the Permalink box in the edit page for that post.

Now when you click on that post, it takes you a page (https://lennyantonelli.ie/middleground) that doesn’t exist.

It now does the exact same for all of my posts, as you’ll see if you try to click on any of them. So essentially every post on the site now directs to a broken page. I did a few searches but can’t seem to find an obvious fix.

Can anyone be of assistance?

Thanks for reading.