The bottom line here is that this is all perfectly normal, and getting out of the mindset of comparing yourself to others takes a long time, even for those who do manage to get it under control. But there are a few things to note, both generally and in your specific situation.
Generally, this is something that most people have to one degree or another, so don't think you're in any way unusual. The flaw in it, of course, is that everyone's life experience is a highly complex mix of all of the things that they have going on in their life, whether that's in relation to their job, health, relationships, interests or anything else. Those things are not only complex but also variable, and people are never either absolutely happy or unhappy with the position they're in. The problem with comparing yourself to others, though, is that when you do that you're only comparing yourself to one or two aspects of what is a very complex picture, and in reality you simply cannot know most of the time of the difficulties that people are dealing with. So the comparison itself is not only flawed because you're not really doing it like for like, but it's a futile exercise anyway because you don't have all the information you need.
To give a couple of real world examples, I'm a barrister and many of my friends are barristers. One of my friends does and always has earned a lot more money than me, because he practises in a very niche area. That's an easy thing to be jealous of, but he's told me on several occasions that he wishes he was more like me because of other things in my life, and the way that I deal with them. As I say, it's easy to pick out the best aspects of someone else's life and be jealous of them, but you need to take the whole package. Another example is another barrister who had a similar practice to me, but has developed his practice into a niche area and is now much more successful in many ways. He's a more natural comparator for me, but he also has significant health issues which makes it difficult for him to do things like play with his children. Would I want to trade his career success if it came with the health issues? No, I would not. So it makes no sense to be jealous of his career success either, and in fact I wish him all the best with that. I've said it before, but these comparisons just do not work. The only thing you can control is yourself. There will always be people who have it better than you in individuals areas, and people who have it worse. But none of that changes anything about you. Focus on yourself, the path you're taking, what makes you happy and what you can do to improve your lot. That's what matters.
On a related note, you say you feel more like a child than an adult. That's because you are. Your brain doesn't stop developing until you're 25. You may legally be an adult, but you are much closer psychologically to a child at the age of 18 or 19. So don't be hard on yourself about being an 'adult'. You still have a lot of developing to do, and that's fine.