As I get older, I find myself making more and more decisions with myself in mind. If I don't see myself happy in the situation, I won't put myself in it, regardless of if it is contingent on other's plans. Living situations, friendships, simple plans for the day all revolve on what I need as a person. If I don't pay attention to myself, who is going to? We've learned when we were younger to always be nice to others and the golden rule, but where does your happiness come into play?
Are you sacrificing your happiness just to make sure everyone else is OK?
There's a time to be selfless and a time to be selfish, and your twenties are the latter. Your twenties are a whirlwind of decisions that often you have to live with for the rest of your life. You decide where you want to live, what kind of education you want, and what kind of person you want to be. Barring lack of funds, your twenties are for you to explore what makes you happy and what you deserve.
They're not for sacrificing time and caring for what other people think.
I'm not saying you must be cold and heartless, or really just a jerk. I'm saying you must protect your happiness, wellbeing and mental health before you are concerned with anyone else. In your twenties you find yourself in so many relationships at once; your close friends, your acquaintances, your significant others. You stretch yourself so thin just to reach out to all of the things you care about. You become attached to people and events that become so habitual that even if they're hurting you, you're willing to accept it.
Don't accept things that are comfy just because you don't think there's anything else out there. Your twenties are most importantly filled with endless opportunities to shape your future. You decide what the rest of your life looks like from your twenties, and would you rather do what makes you happy or what the people around you think you should do?
Self-preservation seems like logic, but being selfish is more than that. Being selfish is being careful who you spend your time with in case they might be draining you. Being selfish is doing what makes you happy at the moment instead of what makes your one friend happy. Being selfish is understanding your limits to social interaction and not being at someone's beck and call.
It's a hard thing to learn to put yourself first, even if it means hurting someone else's feelings or inconveniencing someone else, but it's an important first lesson you learn in adulting. You're the most important person in your life, you make all your own decisions and you have to live with it, so why would anyone else? Choose yourself over everyone else. Give yourself what you need emotionally before giving to others. Don't sell yourself short.