Interestingly, I sorta have the opposite problem. I sometimes can't get "in the zone" until last stock. Sometimes it's nice because it means I can sometimes clutch out a game after taking a serious stock deficit, but ultimately it's something I should probably work to fix. If I could figure out what I'm changing about my play during those high pressure moments that works well, I could potentially be a better player.
And I think this is sort of applicable to your problem too. If you find yourself down a stock or two, really try to think about why you lost those stocks in the first place. Avoid those problems and try to play conservatively and see if you can exploit something they do wrong to bring the stocks back to even.
Sometimes you just get bodied, though. If that's the case, take a deep breath and try not to let it get to you. Just try to pay attention to what went wrong and see how you can avoid those things in the next round. It can be hard, but you have to remember that what happened in the one round doesn't affect the next. Learn from it, but don't dwell on it.