I'd tell myself that life is gonna get really, -extremely- hard for a while and I just have to push through it because there is a light at the end of the tunnel and I'm finally happy where I am right now.
I may also tell her about her future girlfriend {Though I'm sure kid-me wouldn't believe it} and ask her to try to do better& make things work because she is the one; and I really don't want her to fail like I failed.