You were right the first time. Here, I'll prove it.
Could you make the decision at this point to believe otherwise, and truly believe it in your heart?
Nope. One's faith is not a matter of his will.
My belief is shaped by scripture. If I were only exposed to certain Reformed proof texts and teaching, I would very likely believe just as you do.
I did not develop my beliefs from reading creeds like the Westminster Confession of Faith or Calvin's Institutes. As a young Christian I did not know better and pretty much believed what I was told. But as the years went by and I did more personal study, I noticed much scripture that did not agree with Original Sin and other common doctrine. I would study these scriptures carefully and ask God to help me understand correct doctrine. Over the years my beliefs changed.
Now, if you could show me scripture that absolutely supports Original Sin or Calvinism, I would believe it. Scripture is my authority.
That said, I have looked at the scriptures presented as proof texts for these doctrines and do not believe they support OS or Calvinism at all. You would have to come up with something new and pretty special to change my views now. I don't see it happening.