The most common and the biggest sin, according to me, is being ungrateful. Being ungrateful is just a coping mechanism. You blame others, you blame your situations. But- do you realize how lucky you are? Do you realize how much potential you have? Having a life in itself is something to be grateful for. You can pray to God, you have a voice. Ungratefulness makes a person's brain shut down. They constantly have that draining feeling- that something is still incomplete. Something is missing. You got a brand-new phone? But the laptop is so old and trashy! You need a new laptop, too. 

Being ungrateful is self-harm. Some people know that they are very lucky in whatever they have. But they don't have the commitment to stay satisfied. They know that, too. They blame themselves for being so ungrateful. They are aware of their flaws, but can't seem to improve. They try to improve, but some situation (according to them) hinders them from their path. Situations, situations, situations. I know, working on oneself isn't easy. It's actually very difficult. But not impossible. If you think you are at rock bottom, at least you have 9 clouds above you. Take your time, but never stop climbing. The people that demotivate you are correct about whatever they say. Listen to them as those are actually the areas you need to improve. There are actually 2 sides of the coin here. Let me explain. 'You are so poor at time management, that's why you fail all your classes.' Bingo! Work on managing your time effectively. But, 'You will never succeed', 'You are worthless', are some sentences which have a deeper meaning. WHY are you worthless? WHY will you never succeed? Got the answer? It's that simple.