The mound - pitching surface, that you’re working off of when you take the field to actually pitch, can be challenging you in ways that you’re aware of.
Is the surface during your bullpen sessions different from the mound that you use during live fire? Do you feel off-balance and unsure of yourself because of the mound’s surface condition? Are you forced to remain upright a lot - before-during-after your pitches? Is your lower back, neck and shoulders sore after you complete an inning?
On the other hand, being a high school freshman can be a big change for the normal course of action that you’ve experienced. This uncharted waters - sort-a speak, can just be a case of jitters.
Does your game backstop do anything different from your bullpen backstop? Or perhaps your bullpen mate is just playing catch with you… standing up like any position player (fielder), catching the ball, then tossing the ball back.
Are your high school coaches… coaches? Does any one of them watch you, offer advice, help you in any way with this situation?
Ok, to answer your question more directly with respect to performance issues.
When you’re doing bullpen, do you concentrate on being deliberate to various locations? You should concentrate on hitting locations on the player who is catching. Upper right shoulder, upper left shoulder, right rib, left rib, and so on. Then take that same location drill and work on it once you take the mound - only this time your locations should be left shoulder, right shoulder, left knee cap, right knee cap. By the way, the accuracy rate at which you had the greatest success in you bullpen session, should translate the same results on the mound during game time. Use your warmups on the mound to give you a good indication on which location(s) your best at, and which ones you’re not so good at.
With respect to your catcher - if your catcher is moving his body all over the place, from start of finish, this kind of distraction can cause a pitcher all kinds of concern. Also, DON’T USE THE CATCHER’S MITT AS A TAGET for your fastballs. Use the catcher’s body - shoulders and knee protectors. These targets are more pronounced and easier to pitch at.
Now unless you have serious form issues, in general, these suggestions should give you some direction(s).