Ah, honestly, I was half-doubting it myself until I actually saw it with my own eyes like this...
Literal
Ah, I [also-も] actually, like-this witnessing [until-までは] half-believing-half-doubting was [but-が]...
目の当たりにする 'to witness with one's own eyes' is an idiom — 目の当たり 'right before one's eyes.' 半信半疑 is a four-character idiom: 半 'half' + 信 'belief' + 半 'half' + 疑 'doubt.' The trailing が... signals the speaker's realization was overturned by what they saw. オレ is a rough masculine first-person pronoun.