SUBREDDIT: r/relationships
TITLE: How do I [M21] stop worrying about the face that me and my girlfriend [F20] will eventually break up?
POST: I am a guy in his first relationship. I like my girlfriend alot, we get along, & have lots of fun together. But recently, many of my friends relationships have ended and it has caused me to start thinking about the lifespan of mine. Whenever we're together now I always think in the back of my head "How long is this gonna last? Will we be together in 2 weeks? A month? 6 months? Either we're gonna get married or we're gonna break up eventually and this person I care so much about now isn't gonna disappear from my life & wont care about me one day".

One day shes gonna be laying up with another dude talking about "My ex was blah blah & omg he was so blah blah". It devastates me to know that even with all the feelings she has for me now, I'll eventually just be a chapter in her life that she barely remembers, regrets & only brings up to trash me for laughs. It makes me wonder why I even signed up for this.

Obviously this is a terrible mindset to have in a relationship but its just a fact isnt it? I'm a very calculating person and always have to think of every possibility so its always been very hard for me to live in the moment.
TL;DR: 
I constantly live in the sadness knowing that me and my girlfriend will eventually break up. How do you get rid of this mindset and live in the moment so you can enjoy your relationship?