Can't function after being cheated on?

My ex and I decided to give it another go so we got back together. After we reconciled, we basically had sex with each other but as of yesterday, I found out that he is still seeing someone else even after we got back together. He doesn't know that I know. But I'm really shocked because I never expected this from him out of all people. Now I can't manage to sleep (I stay up all night), eat (hardly even eat once a day now), or even study for school (I'm falling back on my assignments). I decided to go back on my anti-depressants as of last night. But what can I do to help get rid of this pain/ anxiety that's not allowing me to function?
Can't function after being cheated on?
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