I am at my lowest low

I am a 23 year old guy who just finished his degree and started a job, but everything feels like it is falling apart. My job is not “the cool thing” I wanted and I feel like my structured hard work for school has not payed off at all. Even worse is that I am also just very ungrateful and resentful to my little sister who seems to enjoy everything and is popping of with her dreams. I realized that I have nothing apart from my studies except from some friends (who are good to me) and it feels like it all to late to make a turnaround, especially because there are a lot of great things to learn, but I am to scared to make the wrong choice. What the hell do I need to do to be more content and actually put in the action to something to make me feel happy, useful, successful and alive again. I am aware that I am also a bit of a whiny, sorry person, but I just keep giving in and that is so not like the person I used to be.