<human>: Hey all!

My high school sweetheart and I called it quits quite amiably six months ago. Now, I've gotten back into the dating game! Exciting! Scary! Fun! Terrible!

I'm spending my summer in Germany, and met a young German girl and we hit it off really well! After about five amazing and wonderful dates, four ending in a...sleepover. And after that, she pulled what my research tells me is the "slow fade." 

**That showed me that dating as a 20-something is so entirely different than in high school (not surprisingly).** I'm still getting over the particular pains of a date fading into the ether, but I think I'm well on my way. 

What I would like to know is what is your biggest piece of advice you would give a young man transported directly from high school dating to full fledged adult dating. (*Bonus points if you have advice on dating German women; I know the general structure but tips would be awesome.*) 

***You have my undying gratitude.

Can you condense the text into a few key points?
<bot>: What is the biggest piece of (serious) advice would you give a young man going from dating at 15 to dating at 21?