can someone explain to me why i'm losing some of my maximum life points after equipping better armors and helmets?

Light armors usually adds health when equipped, if you're switching to heavy armor, you'll lost most of the bonus. I don't think you'll ever go below your base health because of an equipped item, but not sure.

Also, I think Mystic and even Heavy armors (helms usually) can add health (or magic), but Light armors provide the best health boosts.

I can explain this - you have Light Armour currently equipped, which boosts your max HP. Other types of armour (Mystic and Heavy Armour) do not boost max HP. I assume that you're equipping one of these types of armour, hence the loss of max HP.

EDIT: Seru_Kai posted at the exact same time I did, so I'm pretty much repeating what he said. Eh, nevermind.

oh yes you're right, thanks to both of you!