Half of my screen keeps going white, what's causing it?

The problem here is the AMOLED screens cost so much it will quickly total out an old Samsung (unless you get a deal on a minor defect replacement).
You likely broke the screen, which is more common now since the glass is stronger than the display. The problem with Samsung’s unreasonable repair cost is one generation will do it with a cheap phone, maybe 2-3 on an expensive flagship. Beyond that, it's BER in many cases.

Since buying the bare screen will likely total the phone out, your second best option is a choice:

• Bad IMEI donor (match the carrier device class as best you can. Should not matter on the S8, but Sprint was still being quirky on these). HINT: Get one that was blocked by T-Mobile for nonpayment since they block it after 60 days. GSM variants are the same except for the carrier branding, ROM, and bootloader lock signature. However if your T-Mo "finance" donor was a winner like a 6a I intended to use for parts, don't mutilate it. I'm getting it unlocked :-). They are more forgiving about people leaving bad debt than AT&T and Verizon, so that's why you run from these phones outside of use as donors unless you get lucky. IT IS LITERALLY SAFER FOR SELLERS TO DISCOUNT A RISKY PHONE AND SELL IT "AS-IS" AND THEN GET A DEFECT FOR GIVING IT A CHANCE! If the buyer wins the lottery, good for them. The old IMEI will die with the board, so it doesn't matter but move your SIM tray so people know unless it's not engraved there.
  I apologize in advance to the rep who confirmed that my donor 6a probably has a clean bill of health to my surprise. I know these problems all too well.
• Buy a used device (factory unlocked U, no carrier locking). Verizon MIGHT require a "certified" S8, but that's it. Nobody else really cares if it has the the "U" or "A, S, V, P" or not. Ideally, with an issue that kills resale like screen burn. Save the resale agony and get one where it's over with like most of them.

After you find a screen, follow this guide to the point you can drop the new screen into the phone. DO NOT disassemble it further - that breaks these screens so just move the board and cameras, as well as the charge port if need be.