"I was easily led to speak from my heart, ie. (honestly, deeply-felt)." "to give utterance to the burning ardour of my soul" means basically the same as "speaking from the heat of passion of my heart/soul". "with all the fervour that warmed me" basically repeats this another time.
All these feelings in his heart/soul, ie. the sympathy, were evinced by the subject (?the monster. I don't remember who the 'he' is your first sentence was.)
"He evinced certain feelings in my heart and soul that led me to say ..."
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