MacBook migration is stuck

Hey everyone I am upgrading my macbook pro mid 2015 with a new m.2 nvme ssd(which I have already installed a mac os) and was doing a migration from my current HD. I waited like 6 hours and the progression bar was still “finding documents and data” to transfer or something like that so I cancelled it and then it asked if I wanted to shut down even the the transfer wasnt complete so I had it shut down and was going to try and do the process again so I tried starting it up and it looks like its trying to startup with my new nvme ssd(which is plugged in externally btw) and it’s loading for hours on the apple logo and once the bar is fully completed its just stuck there nothing else happens. So then I shut it down manually and hold down option to try to boot from my original HD and when I click on my name to log in I get the circle with the slash through and then a couple seconds later it restarts and tries rebooting from my new ssd

What do I do?

​[Re-Titled by Moderator]