I'm looking for manga where the main character is appears very happy towards her friends but is actually very sad on the inside.

don't know if such a manga exist 😀

Tonari no Shugoshin (well not exactly but kinda)
i am pretty sure there was exactly a series about wat u r looking for but just cant find the name 😔 i will write again when i find it 😕

Kimi no Iru Basho - Real x Fake 😀 yay found it i hope u like it and i am adding more series too maybe u like them...
The Summit
One (not sure again but maybe 😕 )
Dear Friends
Deep Love - Ayu no Monogatari

thanks

Kamikaze Kaitou Jeanne has this. And... that's about all I can think of. It's a somewhat common trait of shoujo manga heroines to hide their inner turmoil and distress and act cheerful on the outside, but KKJ is one series that deals with it significantly (as significantly as a sparkly magical girl shoujo manga can, anyway. but it's pretty good 😀 ).

Fruits Basket
Full Moon wo Sagashite

I just came across another short series like this. 7th Period is a Secret is really good. You should definitely read it if you haven't yet!