I am a young 20 years old who has a plethora of games finished for its age and i'm wondering if it's okay to just switch to another game even if you enjoyed finishing the previous one without doing every single thing that is in the game...

Because i have likely 15-20 real platinated games and dozens of "goldened" by which i mean i made a platinum of a certain part in the game but not every single little thing.

So does it make you a hardcore gamer if you necessary platinate every game you can ?

Personally, I don't think that it should matter if you're a hardcore gamer, a casual gamer, or a tap dancing gamer that juggles baked potatoes in one hand while playing a game with the other hand. The point of video games is to have fun. I don't bother getting a platinum in every game I play. It's like in the days before trophies/achievements. There are games that I would gladly play through multiple times and find every little secret...and then there were games where playing through once and just seeing the main content of the game was more than enough for me...and honestly, there's even games where I simply am not enjoying my time, and just drop the game and sometimes come back and try it again at another time, and sometimes I simply never come back to it at all.

I think you just need to focus on what you enjoy doing. If you're not having fun going for a platinum, then maybe it's time to move onto something more enjoyable. Have fun. Enjoy yourself. If folks say you aren't a "hardcore gamer" for doing that, then tell 'em to go cover themselves in honey and have them tell their opinions to the local bears in your area. The bears will thank you.

you use too much idioms, i don't understand all about what you're saying

Play games whatever way you want to play. Don't worry about whether people think that you're a "hardcore gamer" or not. Have fun.

btw i consider having finished ghost n goblins as awesome even if it's just the fake ending
goes same for games with various endings or hidden real endings like until dawn, clock tower, undertale etc and you ?

Beating a difficult game always feels nice...mostly because you can usually see yourself improve over time before you complete the game. The same goes with beating a game in a genre that you're not great at - I'm not usually great at first-person shooters, for example, so beating BioShock on the highest difficulty without using a Vita-Chamber was pretty neat for me.

By the way, I use idioms and weird descriptions a lot out of habit. If I say something that you don't understand, feel free to let me know. I'll gladly reword it/simplify it. Some of my very closest friends speak English as a second or third language, so I'm used to toning it down for them, when needed.