Agree with Rowen it is about selflove. While she loved herself enought to do whatever she wants and dont feel guilty you might be more mature and lost some of that blind love.
However she gave you less value and perhaps had better social life or higher sex drive. However man Im almost your age and after dealt with two years younger girl I realise how much pain in the but could be 19 year old kid.
Diferences are in lots of importand things - faster change of feelings/adaption/forgeting feelings faster. Also love at that age is more physical than emotional. At that age some girls dont even know clear diference.
Also its best to date your age of girls because they will be more things in common and with that more stuff to talk about. However unwilling to talk about relationship problems is sign she still have to grow up in order to be ready for serious relationship. Also it is a sign that for her there was no so much "us" as there was for you. She simply was disrespectful attention recieving(and taking) immature girl.
However this shit is damaging and you definetly should attent counselling or therapy if you feel like confidence is holding you back from meeting better girls. This have to be talked out with some careful listener for emotional support.
Also she have younger brother so that suggests she used to be dominant and unapologetic with guys. Perhaps she didnt understand or care that what she used to say to brother might be more painful and harder to understand to a guy who actually love and respects her and himself.
Check this out for start and wish you meet more devoted girl in the future.
loveforum.net/threads/85672-Guide-How-to-deal-with-breakup
Doubt kills more dreams than failure ever will