How do you continue with your life when you're depressed?

I've been in a depressive state this month. Every morning when I wake up, my brain constantly reminds me of that I haven't done, what I should've done better and all my past imperfections and mistakes. It was better in the first half of the month, considering it was exam week in my school and I had something to focus on rather than my own negative thoughts. But now exams are over, school has closed down for Christmas, and I've practically lost all motivation to clean up my messy table or do the homework assigned over the holiday. There's this overwhelming feeling of sadness, and I can't focus, and I feel like crying for no reason at all. All I want to do is to lay in bed for the whole day, and mope around about my depression. But I feel like I have no legimitate reason to be sad at all, and I'm not sure if it's just an excuse to be lazy. But the feelings of sadness, hopelessness, guilt and worthlessness are still here. My family planned a trip to Taiwan, we're leaving tomorrow but I don't feel like going and I don't really wanna go and pack.

Long story short: am I experiencing depression or am I just being lazy and ungrateful? And how do you find motivation to continue your daily activities when you're feeling really sad? Now that school's out for the holidays, I don't see any point in putting up a façade. It takes too much energy to do so.