Guys, I need your help to find a manga. This is the description my friend with whom I don't have contact anymore gave me a long time ago (around 6 years ago, so it must have already been published by 2011), so it might not be 100% accurate.

The story plays in a world where first borns possess a special power. The female protagonist is a princess who has been banished from her mother, the queen. The queen later had more children but since they are not her first born children they do not possess a special power, which the royal family has been trying to hide. Anyways, a party is held by the queen and the king (I forgot the occasion but it would seem quite plausible that it was the brithday of one of the kids). The first-born princess sneaks in as a boy and a guard sees her and fell in love upon first sight. Though he was very deeply questioning himself why he would have such feelings for a fellow male. But then an accident happens and the princess mistakenly unleashes her power.

Does this seem familiar to anyone?

I haven't read it but
https://www.mangaupdates.com/series.html?id=56314

Is it this?

Knight Princess ?

Semi-retired.