You can let out a high-pitched voice easily when practicing, if you do rock-paper-scissors at the same time. The sequence of the hand motions don’t have to be exact or in sequence, mix it up. Multitasking with your hands while singing will ease being tense by keeping the mind and body busy.
Many people are afraid and say that, “singing in a high-pitched voice is really difficult.” These negative feelings toward that aspect of singing can psychologically influence your sound production by hardening your larynx and body, which will in turn disrupt your performance in letting out the high-pitched voice. Both physical and mental strengths are necessary for good vocalization.
If you already have the skill to be able to let out a high-pitched voice, just stop extremely concentrating/focusing in letting out the high-pitch voice, and you would be able to let it out easier.
Note1: However, if you on focus playing rock-paper-scissors more than vocalizing, your singing will be messed up. So, please don’t be extreme, think too much, or overdo it.