I really thought this from the bottom of my heart.

Literal

Really heart [from-から] this-kind-of thing [object-を] thought.

心から ('from the heart') is a set adverb meaning 'sincerely, from the bottom of one's heart.' 本当に reinforces the sincerity. こんなこと ('this kind of thing') is slightly vague, suggesting the thought is difficult to articulate directly.