After three attempts, my gallery wall looks like a toddler arranged it

I spent hours measuring and marking where to put the nails. I used a level and everything seemed perfect. But when I stepped back, all the frames were crooked. One is higher on the left, another tilts to the right. I even tried those adhesive strips that claim to hold anything, but they fell off overnight. Now I have holes in the wall and nothing to show for it. I'm about to give up and just leave them as is.