Accept it because you do feel sad, and weird and uncomfortable and it's not fun. Not at all. There's some sort of ache in your chest or shake in your hands, or a thought that's making you feel like your head is flipping the fuck out and you're going crazy. You are probably going to feel like this until something makes you laugh or you fall asleep or your housemate hands you a cup of tea.
Or maybe it will take something bigger to remind you of the way your body can lurch back into happiness, like seeing your family for the first time in months or running far, far away. Maybe you need to cover yourself in dust and sweat and stomp the Earth with a vengeance, then crawl under the dirt when the stars spring out and your body is spent.
It's fine, though.
That's what it seems nobody ever talks about. It's fine to feel sad, you'll move on. Emotions are strange complex things which affect our glands and nerves and make us hunch over and render us incapable of proper thinking or communication, but they pass. They move through us if we're prepared to let them go.
I think when you're feeling sad the most important thing to do is to be fine with it, and then remember that you also have the ability to feel happy, you are capable of that even if right now you feel incapable of everything.