i think both meant something like in the middle/center, what's the difference ?

Think about a person and one stick. Then, imagine the person grabbing the stick horizontally and centered so that he/she remains balanced (like a tight-rope walker), in that case, you have 央 (note the similarity with 人, person and 一, one horizontal stroke)

Now, imagine the case of a stick that passes right through the center of the person, and exits through his/her mouth (like an impaled person). In that case, you have 中. (note the similarity with 口, mouth, and |, the vertical stroke).

In other words, 央 means "center" as "well aligned, median", and 中 represents the "middle point". 中 also means China, because Chinese viewed their country as the center of the world (which was a flat disk)

中 can also just be 'inside', 'among', or 'during' (when referring to an action over time), with no real specification of it being the exact center, which might help you differentiate between the two meanings better.

中 is used everywhere, while 央 is used in a less casual way, only appearing in some particular word combinations.

Strange that you brought this up at the very time i was reading some poem to my little niece.

蒹葭苍苍， 白露为霜， 所谓伊人， 在水一方。
溯洄从之， 道阻且长， 溯游从之， 宛在水中央。 (水中央=middle of the river)