One day? That's like giving a kid ice cream, only to take it away as soon as it touches his mouth.
My first reaction might be to do anything to stay forever. After all, if I got in, there must be some way to stay in. I would try to find some way to anchor myself to Equestria. If Celestia can't help, maybe Twilight can. I'd try to find her.
On the other hand, I might try to make the best of my time there, instead. If I failed to stay, then I'd have thrown away my opportunity to experience utopia. In this scenario, I'd try to hang out with Twilight and the Mane 6, and enjoy what I can, taking everything in and preserving them in my memories.
Sure, I'll probably be depressed as hell and completely disillusioned with my world when I get back, but I would at least have known paradise. I probably won't be suicidal; instead, I might become obsessed with returning to Equestria. I'd try to become immortal, then try to develop a means to return. Maybe I'll just go mad. Or maybe I'll just accept it.
I mean, who knows, perhaps it's better to go home. Perhaps the experience would be an inspiration to improve this world. Perhaps Equestria's actually a dystopia lead by a ruthless dictator. Besides, I can't just leave my family and friends behind just like that. I can't leave my responsibilities. I can't leave my troubles to others. It'll be wrong if I leave. Earth is home, and I'll just have to accept that.
That is, until interuniversal travel gets invented.