Control. It's all about control.
Bruce is sick of it.
Day in and day out it's the same – hold back, don't take risks, don't let yourself get too close to anyone and above all, do not lose control. Keep cool, keep calm, and hold the monster back. Don't let him hurt anyone.
Stay in control.
He hates it. Hates his life, hates himself, and above all hates that thing that has been seizing control of his body every time he so much as lets a vein twitch in his forehead. He hates how there's no way to stop the Other Guy whenever he decide to take his body for a joyride, no way for him to save the thousands of innocent people who flee in terror at the sight of the death machine that barrels through them. He hates how he takes so many more lives than he saves, and he can't do a damn thing about it.
It gets easier. Eventually, after years of therapy and yoga and various different coping mechanisms that he uses to keep a lid on it, it gets easier. The anger is always there, seething, bubbling, ready to burst out at any second, but he learns to manage it. Bit by agonizing bit, he learns restraint. It doesn't get any better, and it definitely doesn't make it any less painful, but it gets easier.
But it's not perfect.
He tries. He tries so freaking hard but he just can't stop himself sometimes. The other guy has strong self-preservation instincts – that much is painfully obvious, and his carefully constructed barrier of calm and composure can't bear the brunt of the monster's desperation to survive.
He hates it.
He hates himself.
He wishes that he could just kill himself and get the entire thing over with. But it's impossible.
He's about to break. He knows it. Betty knew it. That's part of why she left – she couldn't handle it, couldn't handle him, no matter how hard she tried, no matter how supportive she tried to be. This wasn't what she'd signed up for. Heck, it isn't what he signed up for either, but unlike his ex, he doesn't have the option to just turn and walk away.
He's stuck with it for all of eternity. She isn't. End of story.
It's all about control.
Control that he just doesn't have.
And Bruce has had enough of it.
(It isn't until years later, when Natasha Romanoff learns how to bring him back to the real world while the Other Guy has the wheel, that he dares to think that things could ever get better.)