In case you need a reminder, here's the Hoovering thread.
Hope is a devil of a thing - it led me to telling myself even bigger lies than he did.
If he had true remorse it would not be contingent on R or you even considering taking him back. Remember there is nothing stopping him from ending it with OW and started to do the work to fix WTF ever is wrong with him right now.
You're right - you should not be his Plan B.
This isn't a YOU problem or an M problem - this is a him problem. Leave him to his consequences now - he is choosing this every single day. Stop talking to him - for your healing and for your sanity.
Ignore his words - watch his actions.
Look how pretty their words can be? It almost disguises their ugly actions. Almost....