、。
It seems the last person I shared my thoughts with thought I was crazy.
Literal
I [subject-が] last [target-に] own [genitive-の] thoughts [object-を] told person [topic-は], me [object-を] crazy [copula-だ-quotative-と] thought [seems-ようだ].
A multi-clause sentence with a long relative clause modifying 人: '[the person I last told my thoughts to].' Inside, 僕が is the subject of the relative clause (が, not は, because the topic は scopes only to the matrix sentence). The ようだ at the end marks the whole thing as an inference — 'it seems that.' Note that 気違い (kichigai, 'crazy person/lunatic') is flagged as 'sensitive/derogatory' in modern Japanese — it's a slur for the mentally ill and is largely avoided in contemporary speech outside fixed phrases or self-deprecation; learners should recognize it but use 変な人 or おかしい instead.