What it all comes down to is three little words: TRUST YOUR STUFF!
When you're warming up prior to coming into a game, whether it be as a starter or as a reliever, check out your pitches to see what they're doing. Sometimes you'll find that one pitch or another isn't working the way you'd like it to---the slider might be flat, or the knuckleball just won't knuckle, or your fast ball isn't responding the way you'd like. Okay, leave that particular pitch alone for the time being and---especially if you throw a lot of "snake jazz", use it. You might also try a different arm angle; you might be more comfortable with a low 3/4 or even a sidearm delivery.
I remember a story about a pitcher the Cincinnati Reds had once---a guy named Jay Hook, who apparently didn't trust his stuff. He started against the Pirates, and they ate him up from the beginning, converting every pitch he threw into line-drive extra-base hits. When he was taken out of the game in the fifth inning, he returned to the dugout and sat in a corner bemoaning the fact that his fast ball had deserted him: "Without my fast ball I can't pitch." Jim Brosnan, the Reds' relief ace (and he might have made a good pitching coach if he'd wanted to), tried in vain to explain to Hook that a situation like this was when you learned the game. "You have other pitches to throw; use them when your fast ball isn't there". But he might as well have been talking to the wall; Hook appeared not to hear him but continued to moan and kvetch "Without my fast ball I can't pitch." The guy didn't last long in the majors after that.
This, I believe, is one of the keys to success on the mound---trust your stuff and use it. And you don't have to strike everybody out. They talk about "pitching to contact"---just another way of saying what my pitching coach told me years ago: "Get the ball over the plate and MAKE THEM HIT IT. Make them go after YOUR pitch, what you want them to hit. And you have good fielders behind you; let them get a few outs for you."