How to reset achievements after u got an achievement u weren’t suppose to earn

Nope. Once you get an achievement, there's no way to remove it.

^ I think that what the op ment by. I think he or she means by view achievements. It can be handey if u where compairing you with someone else. But to see the achievement u just earned. I can tell u and that said person there is a message what u earned.

Just for clarification: You can play the first area (Earth) on any difficulty,
BUT, if you're after the Insanity Achievement, you will need to change the difficulty to Insanity
BEFORE you leave Earth and not change it again. Changing it to a lower difficulty even for one mission, will negate earning the achievement.

Also i would like to know if, when u reset the gamerscore, will the achievements i have already earned, not be earnable again?

See the only thing why you dont have ur achievements are bc u recovered uraccount after u earned these achievements offline so thats the only thing. You cant fix it youhave toearn the achievements again it warns you you will lose achievements if you recover
ur account and dont sync them with online. Sorry.