What you said is not biblical. Luke 12:42-47 Christ gives us the story of the servant who was put in charge by his master. This servant became forgetful of his masters return and began doing things he knew he shouldnt be doing, and on the masters return he was cut up and given a place with the unbelievers. There are other verses in the bible about His followers being given a place with the unbelievers as well. So yes, Christ does teach us that those who go back to living in sin can be cast away from Him.
Did Jesus pay for every single sin or not?
If so, your whole argument falls apart.
If Jesus paid for all sin, there is nothing left to atone for.
Nothing.
You say Jesus taught that those who live in sin can lose their salvation.
How many sins will damn a person?
One, single sin.
So, if you commit even one sin after conversion, you've lost your salvation.
Now tell me....how many times a day do you lose and regain your salvation?