if I press it once it'll turn back on until I continue to press it until it shuts off
Is it stock or custom ROM ? Some Roms have tweaks to the kernel that may cause issues with the device sleeping or when you turn them off. Depending of it you are stock or running a custom ROM there are several things you can try to eliminate the software as the issue.
Reinstall using one of the nexus toolkits to download and flash the official Google sick image replacing the partitions, boot loader etc.
Use a different ROM and try your luck.
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