I must be doing something wrong

I've been playing for about 5 years now I think, solo; I've gotten to as far as defeating WOF but then I never played past that bc I thought I missed a lot of pre-hardmode stuff so I started new worlds but never got as far as that save. Stopped playing for a while but recently got back into it after the update.

All that to say: Why can I not defeat these bosses? Took me like 5 tries with BOC, I cannot even get close to defeating skeletron. Crimson Armor, potions, heart lanterns, bast statue, Blade of Grass... I just suck at fighting? King slime also took me like 3 tries to beat. Additionally, I just feel like I'm missing a lot of this game bc I don't understand y'alls loadouts, I don't get how you make these beautiful fantasy worlds type structures, I don't recognize half the crafting stations...

So, what should I be doing in this game? I defeated king slime, goblin army, BOC, mined meteorite, have chests full of loot, reforged items, found islands, found shimmer, I'm at a loss and every post I see here makes me feel like I'm not playing this game correctly.

TIA for any help!