If you lost someone you really important in your life it makes everything you do feel useless and nothing matters to you.

You only wish to be with that person. If you want to meet that person in haven again, then suddenly your life seems so long and you feel like you will have to wait forever to meet that person again. In that moment, you only have to think of what that precious person would tell you right now. Would the person you lost like to see you broken and down? Would it make that person happy that you are feeling yourself worthless because of him/her?

I might not know who you lost, but I bet that the person would not what to see you like this. If not for your parents, your friends or yourself, then at least stand up again for the person you lost. Make his/her dead be not for nothing. Yes, you can mourn. But don't mourn your whole life. Think of it like that the person is in a better place now, without pain, without worries. His/her only worry would be, what happened with the ones he/she left behind? Let that person rest in peace by being strong for that person.