Complex sentence: After initial doubts because of her uncanny likeness to his enemy, Gilbert ultimately became convinced she wasn't Lia, grew to trust her, slowly fell in love with her and ultimately stirred his heart to beat again as his powers temporarily weakened.
Split into simpler sentences: After initial doubts because of her uncanny likeness to his enemy, Gilbert ultimately became convinced she wasn't Lia. He fell in love with her, his feelings stirred his heart to beat again as his powers temporarily weakened.