(Kindly chat with me before selecting me so that I can assure you about the changes you need)
Hey there! I assume that your website is made by Wordpress,
I can make something like when your customer selects Proceed to Checkout Button,
I can show above the checkout form something like "Returning customer? Click here to login" and if not then the customer have to enter password at the bottom of the checkout page to crate a new account
"SO THE CUSTOMER DON'T HAVE TO LEAVE YOUR CHECKOUT PAGE TO LOGIN OR REGISTER"
(so you will not lose any of your customer)
I can show you the screenshots how will I do it, and also other minor things, just chat with me to see the screenshots or to discuss about other minor changes you need.
(As I am new freelancer I am charging less)
Have a good day
Regards,
Harshil