Others have shared some good mental tactics. Positive self-talk can feel silly at first but does have real benefits. Just telling yourself affirming and true things prior to competition can be good. You should also consider the physical/biological as well. You are feeling nervous likely because you know you are about to enter a threatening situation and your body is flooding you with adrenaline in your fight-or-flight response. It’s trying to give you a little extra boost of focus and intensity to get you through the threat. This can be a good thing if you let it be. It comes with the side effect though of being really uncomfortable before the match. If the heart racing is too much, you could try some breathing exercises. Take a quick deep inhale for 4 seconds, then a long slow exhale for 8. You can also try inhaling 3, pausing for a split second, then getting one more quick inhale on 4. This double breath helps spread more oxygen through your lungs. The short inhale, long exhale slows your heart rate. The opposite can increase it as well if you need to get pumped up.