I never thought about actually entering a relationship until I graduated college (I'm 23) because I had the same thought process as you! The thing about my friends though is that they're all in serious relationships now and I don't see them often at all. It was an extremely hard year for me because it took me a long time to figure out that they didn't abandon me or stop being my friend. They just took a natural next step in their lives. I shouldn't be angry with them for being happy in their relationships and spending time with their SO's over me. It was just a slap in the face for me because I had never been in a relationship so I literally couldn't imagine putting a boyfriend above my friends. It was a childish way of thinking and I wish I could go back and tell my past self that. Luckily I never brought my frustrations to my friends since I tend to deal with my emotions internally and I always felt like it sounded so crazy to be like "you stopped texting me every day! Now we only talk once a month at most, and you never have time to hangout with me! I've seen you 4 times this year!) and it is crazy. You will not be as important to your friends when they enter the adult world and their time is limited, not because they don't love you, but because they have to prioritize their relationships.

I feel like your me in your post and this is what I would say to my past self:

Stop focusing all of your energy on your friends! Start looking for a relationship!!! At the end of the day, your friends are all going to go home to their SO's and it's not something to be resentful of. You don't try and you need to start so we can avoid this emotional shitstorm that's about to happen to you.