First, your nervous energy is COMPLETELY NORMAL! Don't think that it is unusual at all. I remember very vividly having those same feelings as a high school player. After getting to college I would feel much the same way if I was struggling at the plate and I would get nervous about another strike out. Because I knew I was fighting for playing time everyday, I started feeling nervous during BP as well. I believe that this happens to everyone at some point if they play the game long enough. There is no question that controlling these feelings can be challenging, so I will only give you three relatively simple things to try at the start:
1. Focus on a deep breath before getting into the batter's box or toeing the rubber. Breathing is important because it can slow the heart rate and relax tense muscles, but it also get a play back to what is happening right now. People don't spend time thinking about their last breath (in the past) or worrying about their next breath (in the future). They focus on their current breath - in the present...and that is KEY for controlling nervous energy.
2. Have a cue word or very short phrase that gets your mind back to a simple part of your swing that you know helps your movement pattern. "Back hip"..."Stay over the rubber"..."Explosive!" These short phrases will get you focused on what you need to do to be successful rather than worrying about the outcome if you are not successful.
3. Reframe the feelings as positives. The butterflies that some people get before games are actually a natural feeling as blood gets diverted to the working muscles and away from the digestive system as the body prepares for competition. Reframe these feelings to be a great sign that your body is preparing for game time!
Off-season training or a practice situation is a GREAT place to try out some of these simple techniques and start to implement them as part of your hitting or pitching routine.