It would be hard to say that someone who is repeatedly manipulated--and manipulated into assisting in committing murder--is true to himself, thus he cannot "stay true to himself." One has to know one's own mind and have inner integrity of principles and right-mindedness to be true to one's self. Cal has none of these things. He is swayed by guilt. He has done wrong and knows no way of addressing it to set it right, thus crumbles under a load of guilt. He has left a woman widowed and knows no way of addressing that except to obsess over her, thus creating (the illusion of?) a romance. He continues to be swayed and manipulated by people he doesn't believe or admire. He can find no way to liberate himself from their power.
Within this description, two steps Cal does take toward self-understanding and the self-expression of being true to self are suspect as being the result of inner psychological manipulation. Specifically, the idea Cal tells himself that his relationship will deepen with Marcella if he confesses can't really be considered being true to himself because it ignores the psychologically skewed foundation of their love affair. Similarly, his expressed desire to be severely punished, which he contemplates while waiting to be arrested, can't be thought of as being true to himself because it comes from a conscience that has been manipulated as firmly as his actions have been.
They gathered [berries] for half an hour, talking and joking. The happier Cal felt, the sadder he became. He wanted to confess to her, to weep and be forgiven. He saw the scene in his mind of her holding him, comforting him; [then] he saw the scene as he knew it would be in reality and it horrified him. [...] It was worse than he thought. The slightness of her skin made his confession impossible.
One moment might reflect a glimmering of true knowledge of self. When Cal makes the anonymous call about the bomb, he may finally have found a peek-hole into his own beliefs and conscience; therefore this may represent his being true to himself. This peek-hole is obscured again as he wrestles with the profound guilt that equally manipulates him and continues to leave him devoid of self-knowledge and, therefore, self-truth, without which, none can be true to themselves.