I feel like I'm not really an adult because I've never been in any kind of romantic relationship
What makes you think that being in a romantic relationship will make you an adult?
Because most of my friends and family members in the past were in relationships and they seems so happy and in love.
We tend to get envious other couples when something about their relationship is missing from our own. Talk to people and get to know them so a romantic spark can happen for you without being forced.
I don't want to talk to people. I'm not ready for a relationship right now.
You don't have to be in a relationship now if you don't want to. It is also okay to be jealous about your friends relationships, just be careful about telling them that.