Ya can't. You have to do it at purchase. You can always cancel insurance, but you can't add it later.
"I'm not trying to be a **** so don't get offended by this, it's just my first reaction. Ahem. You're stupid for not getting insurance on an expensive phone." You're funny
Anyway, dude I'm so sorry. I vote for "sell it for a loss and get a new one". I don't see any other choice. Repair shops are really great at fixing iphones, but they couldn't fix my old S5 properly, and that didn't even have a curved screen. Cut your losses and never make the same mistake again.
Some more free advice: Next time someone needs a flashlight, hold the phone yourself. And don't leave it on a counter where a cat can knock it off. And don't leave it on a patio chair where a child can run into it with a bike. Ask me how I know this! lol. Yes I finally wized up and got insurance on my Edge when I bought it, I'm extremely choosy about where I put it down, and I try not to hand it to people. I'm trying to always remind myself: "This is a $900 computer. Do I really want to put it there/hand it to that person?" Remember, while it's certainly better to have insurance and be able to get it replaced, the replacement won't be a brand new phone, but will be a refurbished one that might have god-only-knows-what wrong with it.