Authentication error connecting mqtt to IOT core

I am getting authorization failures according to the monitor

What "monitor" are you talking about?

Please follow these steps to validate the cloud part, and confirm a correct certificate and key:

https://docs.aws.amazon.com/iot/latest/developerguide/diagnosing-connectivity-issues.html

https://repost.aws/articles/ARKdSDxoQSTpOPL8ZblXIULQ/how-to-troubleshoot-tls-connection-issues-with-your-aws-iot-core-endpoint

only gave me outdated mbedtls instuctions that seem to use deprecated functionality in the mbedtls library (pkcs11)

It's likely not outdated. PKCS11 is an API to use when storing the certificate and private key in a hardware security module.

To use mbedtls without PKCS11, here's an implementation: https://github.com/FreeRTOS/FreeRTOS/blob/main/FreeRTOS-Plus/Source/Application-Protocols/network_transport/transport_mbedtls.c

See the setCredentials function for how to set the client certificate and private key.

This is one of many abstraction layer implementations: https://github.com/FreeRTOS/FreeRTOS/tree/main/FreeRTOS-Plus/Source/Application-Protocols/network_transport

These layers are called into from this demo, for example: https://github.com/FreeRTOS/FreeRTOS/blob/3b34f8028dae6c3e17e37c517c0215f9d850711f/FreeRTOS-Plus/Demo/AWS/Mqtt_Demo_Helpers/mqtt_demo_helpers.c#L455