Yes, it is impossible for a Christian to know with certainty what God wants. We live the way we live based on faith that we are understanding God's scripture and God's communications with us correctly. I will never claim to have figured it out, nor will I believe anyone who says they have.
"Sin is not real, but only subjective."
WRONG. Scripture is clear about what is sin and what is not.
Will all Christians and supposed Christians ever agree? Absolutely not.
Incorrect, but an interesting thesis.
A thesis which you immediately contradict by stating the following:
Scripture is clear about what is sin and what is not.
Correct.
Your position right now amounts to: "God's will is completely unknowable, except when it comes to how He wants us to act."
Scripture is clear about what is sin and what isn't and it is also clear about what God wants.
In fact, Scripture is nothing other than God telling us in human language what He wants and what He is doing to accomplish it.
Will all Christians and supposed Christians ever agree? Absolutely not.
Yet you just said that Scripture is clear about what is sin and what is not.