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But God's decree in no sense is the ultimate cause of my sinning, else would be guilty of being the Author of Sin, which all but a few individuals throughout history, even the staunchest supralapsarians, have categorically denied. Man is totally responsible for all his choices, AND all man's choices are made freely, AND all man's choices have been foreordained/decreed by God from eternity.
In that case you're clearly not a Calvinist. All of the Reformed standards say that God is the ultimate cause of the sins of the reprobate--without being the Author of sin.

I can give you lots of biblical examples:

So Absalom and all the men of Israel said, "The advice of Hushai the Archite is better than the advice of Ahithophel." For the LORD had purposed to defeat the good advice of Ahithophel, to the intent that the LORD might bring disaster on Absalom. (2 Samuel 17:14 NKJ)

God caused Absalom to follow the advice of Hushai rather than the good advice of Ahithophel.

God opened the heart of Lydia:

Now a certain woman named Lydia heard us. She was a seller of purple from the city of Thyatira, who worshiped God. The Lord opened her heart to heed the things spoken by Paul. (Acts 16:14 NKJ)

and the disciples:

And He opened their understanding, that they might comprehend the Scriptures. (Luke 24:45 NKJ)

But he "hardened" others:

And the LORD said to Moses, "When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. But I will harden his heart, so that he will not let the people go. (Exodus 4:21 NKJ)

God also sends evil spirits and delusions:

And it came to pass on the morrow, that the evil spirit from God came upon Saul, and he prophesied in the midst of the house: and David played with his hand, as at other times: and there was a javelin in Saul's hand. (1 Samuel 18:10 KJV)

And for this cause God shall send them strong delusion, that they should believe a lie: 12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness. (2 Thessalonians 2:11-12 KJV)

God is therefore the ultimate cause of both faith and unbelief.

Charlie

For "who has known the mind of the LORD that he may instruct Him?" But we have the mind of Christ. (1 Corinthians 2:16 NKJ)