I am in my early 20s. I have had a few boyfriends, a few more flirtationships, the like over the years. Some may even call my dating life up to this point a kind of serial monogamy, I'm not ashamed. I have had a few flings I knew wouldn't last, but for the most part, I am the type to date seriously. That being said, when I do fancy someone, I usually fall pretty hard for them relatively fast.
Many friends throughout the years have warned me, reminded me of my past, told me to slow it down, the advice really never stops. But, alas I truly never listen. I allow myself to fall deeply for people as quickly as my heart desires because life is more enjoyable this way. Have I been hurt a time or two? Of course, I have! But I would much rather enjoy the now and learn from it later than constantly be concerned that what I have going isn't going to "work out". What does "work out" mean anyway?!
For a relationship to "work out" doesn't mean it has to follow a specific path and end at the same point as every other relationship. Sometimes, the best relationships aren't forever, but they still make one hell of a chapter in your book of life. Being terrified that life is always going to bite you in the ass is no way to actually LIVE life, and a life without living is a life not worth living.
So, stop being worried that they're into someone else, that they don't want the same thing as you in 50 years, that they don't like you as much as you like them. Just be honest with everyone about your feelings, and live your truest, happiest life! I promise, once you stop being afraid of falling, the jump feels even better.