First: a girl (or woman) cannot, may not and must not give 100% of whatever you may want to call it, heart or mind or trust or whatever, to one boy (or man). I believe that my own mother is the first person to take into account for being the source of my first rule. But then again, what good is a rule if it doesn't have an escape clause? 100% is acceptable for certain period of time. Afterwards, take some back, they're yours to keep.
Second: girls are too complicated. Too complicated is never nice. Avoid being one. But bear in mind that every once in a while, complication is necessary to remind some people around you that you're absolutely not the person they would like to mess around with. The reasonable ratio is probably 30:70. Feel free to determine yourself which one is for which.
Third: do not (ever) try too hard for anything in this world, but try hard enough. You know your limit, honesty is the key (including to yourself).
I write this as I listen to When Did You Fall (In Love With Me) by Chris Rice. I should've listened to Mrs. Cold by Kings of Convenience instead.