Bullpen sessions are different than game action. It sounds obvious, but many guys beat themselves up during their practice sessions when things are off or they feel less than 100%.
Always remember that a bullpen session is part of a pitcher's "routine" - good or bad - and it is often the one thing that keep a pitcher's sanity during the season. Much like getting in some running or hitting the gym for a lift, bullpen work is a training session before the next competition. If things are going well, it is all about repeating what was done last session. Slump? Time to work on what is feeling off at that particular time. I know in my own experience it was usually trying to either maintain or regain command of my secondary pitches, depending on how effective they have been.
A bullpen session is the time to work on the whole pitch arsenal, keeping the radar readings to a useful minimum to check and see what the speed differentials might be. Velocity is usually developed in the off season and into the preseason. During the season, there will be fluctuations in velocity - just watch your favorite MLB pitchers from game to game, they can have 3 to 5 mph variations at times. Your command of the off-speed stuff will be there for you always if you regularly work on it, and this is what a coach loves to see.
Use all of your pitches with confidence! I can't stress the importance of changing speeds regularly.
The term is not "changing strikes", but "changing speeds". Off-speed pitches can be effective even when they are not perfectly located. A change in the dirt will often get the hitter aggressively leaning in over the plate, making the next fastball look 3 to 5 mph faster following the off-speed in the dirt.
It is not so much "how" you throw your off-speed pitches as much as "when" you throw them. This point can go against a young pitcher's mindset, especially during practice sessions. A belt high slider is not the prettiest sight for a pitcher. Believe me when I say I threw my share of high sliders that have gotten drilled into the gap or into the bleachers. But there have almost been as many that were swung through, taken for a called strike, or softly popped up after getting the hitter a little off balance. That bad high breaker may have been a first pitch, "get me over" type where the hitter was looking for a fastball only, so no swing. Or there may have been a case (and it happens a lot) when the slider was so bad that it didn't break and the batter literally swings under it, where he thought it was going to end up. Yes, a pitch can be so bad that is good! So, when you are in your bullpen and the secondary pitch location is a little bit off, try to tweak it but keep in mind that you need the confidence to throw it at any count (the ultimate goal) and if it is a high strike it still could be ok in a game, so move on and don't over think it. You will find the right location more often than you won't.
And sometimes, great pitches with perfect location get hit hard. If a hitter is looking for that great curveball you might have that day, he will find it and hit it hard. That's what makes baseball such a great test for your patience. Things have a way of evening out!
A pitcher's goal is to put pressure on the hitter with all of his pitches, making it harder for the hitter to be aggressive with his timing or pitch recognition. Coaches will find it hard to give a pitcher meaningful innings if he doesn't have the ability to use his pitches with conviction and confidence.
Is velocity important, absolutely! But as you continue to get better at the game, take what your body gives you, throw what is working best when you are out there, and continue to practice getting all of your pitches over the plate. Before you know it, everything will come together for you and success will be there.