When you grow up as the wrong assigned gender, you are going to miss out on a lot of things that should have been available to you if only people had known. Sleepovers, camping trips, girl/boy scouts, shopping trips, cheerleading or sports. Events that are co-ed may have very different feelings attached to them based on how you engage with them, like going to prom, religious ceremonies (such as having a bat mitzvah instead of a bar mitzvah), and even just the act of courtship. These dysphoria may also be biological in origin, such as a sorrow over having not given birth to or breastfed your children.
These missed opportunities can manifest as feelings of loss and hurt. Furthermore, the memories of things you did have access to but wouldn’t have otherwise, or events that were performed in the wrong gender, can also be a sour point, as these may have awkward attachments. Imagine having to be a groom at your wedding when you know you should have been a bride; growing up dreaming about your perfect wedding, and then playing the wrong role in it.
Sometimes historical dysphoria can manifest existentially, hitting you with all the grief of the youth lost. All the dating, the teenage antics, the parties, even just having been able to be sexual with the correct parts while your body was young and you had no responsibilities. It is time that can never be gained back.
Many trans people attempt to recapture some of these lost events, hosting or attending queer proms, organizing sleepovers, performing vow renewals with their spouses, and engaging in common puberty rites of passage like having a mother figure help them shop for their first bra, or having a father figure teach them to shave. However, ultimately, historical dysphoria is something that can never be relieved. You can make new experiences to replace the ones you lost, but you can never turn back the clock.
This is one of many reasons why affirming trans youth is so important. Boys want to do common boy things and girls want to do common girl things, and when they miss out they will not forget.