Letting go, getting up and going ahead
We become our thoughts.
To know what you will become, you need to know your thoughts. That's passive. To become better in future than what you are right now, you need to have better thoughts. That's active.
The thoughts that influence your being aren't the ones that come when you are in office, busy working; or when you are playing that soccer match with 2 minutes left in game. You are busy in something at that time. You are concentrating on something. The thoughts you should be improving are the ones which come when you are travelling alone. Or when you are staring at the ceiling fan trying to sleep. Changing those to better ones will help you become better.
You ask how to improve the thoughts? They aren't like a skill which we can improve? They aren't like a physical object that can be polished and made shiny and attractive again. True. They aren't anything like that. But they are a reflection of your feelings; feelings are for other people. Most of us miss someone, someone who isn't with us right now, maybe due to our actions, their actions or divine actions. That's a fact. How we interpret those feelings will reflect in our thoughts.
There are some ways to interpret feelings positively:
Never think ill for someone : We often call people as 'individuals'. That means they are independent. Independent to think, act and live. Even though the people we interact with, often take decisions taking us into account. It need not be the case everytime. Sometimes they think independently, which may not do us good. That's fair. It's their choice.
Put yourself in other's shoes : Whenever anything doesn't go by your way, take two minutes and think from the other person's perspective. I wasn't called at the party by the person I thought to be my closest friend. Okay. I'll think from his perspective. Why I would have done this? Maybe I was busy? Maybe I forgot? Maybe I didn't want to invite me for some reason? You might find out the real reason of the incident.
Improvise yourself : Hate eats you from inside. There's nothing more fruitless than hatred. It does nothing in material world. It's all in your mind and it poisons your thoughts and in-turn your actions. Truth gives you power. Accepting things that they are, gives you peace. It gives you a realistic picture of the world around, rather than some illusion. This was so well pictured in this scene from movie - Abraham Lincoln: Vampire Hunter.
"True power comes not from hate, but from truth."
So let go of whatever thoughts you think are holding you back from your progress. That'll take away the hate. Accept the truth, even if you are far from your goal. That'll give you the power. Now that you have the true power, you can go ahead and achieve anything you wish!