What Lessons Can You Take From This Movie(Relationship Wise)

I've seen Before Sunrise, waited for Before Sunset. This film is for me in between, waiting for the third... As she said in Before Sunset (singing her song at the end) She had many relation, but could never forget Vienna as this was unfinnished.
Let me sing you a waltz / Out of nowhere, out of my thoughts / Let me sing you a waltz / About this one night stand / You were, for me, that night / Everything I always dreamt of in life / But now you're gone / You are far gone / All the way to your island of rain / It was for you just a one night thing / But you were much more to me, just so you know / I don't care what they say / I know what you meant for me that day / I just want another try, I just want another night / Even if it doesn't seem quite right / You meant for me much more than anyone I've met before / One single night with you, little Jesse, is worth a thousand with anybody / I have no bitterness, my sweet / I'll never forget this one night thing / Even tomorrow in other arms, my heart will stay yours until I die / Let me sing you a waltz / Out of nowhere, out of my blues / Let me sing you a waltz / About this lovely one night stand.”

I find it depressing what she says in the end of the 2 Days in Paris.
Marion: ”It always fascinated me how people go from loving you madly to nothing at all, nothing. It hurts so much. When I feel someone is going to leave me, I have a tendency to break up first before I get to hear the whole thing. Here it is. One more, one less. Another wasted love story. I really love this one. When I think that its over, that I'll never see him again like this... well yes, I'll bump into him, we'll meet our new boyfriend and girlfriend, act as if we had never been together, then we'll slowly think of each other less and less until we forget each other completely. Almost. Always the same for me. Break up, break down. Drunk up, fool around. Meet one guy, then another, *beep* around. Forget the one and only. Then after a few months of total emptiness start again to look for true love, desperately look everywhere and after two years of loneliness meet a new love and swear it is the one, until that one is gone as well. There's a moment in life where you can't recover any more from another break-up. And even if this person bugs you sixty percent of the time, well you still can’t live without him. And even if he wakes you up every day by sneezing right in your face, well you love his sneezes more than anyone else's kisses. ”
Just this she tells him in Before Sunset.... The unfinnished Vienna-thing interfeers her relation (and his). Because of this she jumps around, quarrels with him, her parents, sister, the taxidriver ...