This could be super useful! As a writer (who writes a LOT), here's some of my tips:
Keep a sample of dialogue for each important character. This can be incredibly useful for reminding yourself of what kind of a person they are. For example, one character from one of my favorite video games has the line "Consume the darkness, return it to light". This tells you everything you need not only about the character, but how he speaks. He is a man who faces darkness in an effort to turn it good once again. You won't believe how many times I've had to write dialogue and something feels just a bit out of character, so I return to the line they're assigned to refresh myself on who they are.
Take your time. When I first started to write, I rushed every single chapter. All the time. I was so eager to get to the good stuff, that I sloppily sped through everything else. This undermines those epic moments you're trying to create by bringing your audience on a roller-coaster with no breaks. Calm down. Savor the small moments, because they emphasize the big ones.
Comedy is all about the characters. Throwing this one in there because sometimes people are daunted when faced with writing a comedic moment. But it's actually rather easy when you realize comedy stems naturally between character interactions. The way I think of it, take two characters, put them in an elevator. How do they react? What do they say? What do they do? Usually this jogs something funny. Hardly anything worth writing about comes from just one character.
DO NOT kill a character for shock value. This one is done far too often. Ask yourself: Does killing this character advance the story? Does it develop another character? Does it raise the stakes? If no to any of these, they probably aren't worth killing. A good example of a bad death is (don't kill me) Saw Gerrera from Star Wars: Rogue One (Google him). His death was...pointless. Erso didn't change from it. The plot didn't need it. The dangers would have been just as dangerous with him alive. So all in all, a wasteful death.
And whatever you do, please, I beg you, DO NOT ROMANTICIZE A MENTAL ILLNESS. I once read a story on Tapas where a boy killed himself at the very start of the story and that actually made him stronger. Do not do this. Do not make mental illness seem cool, or romantic, or use it to make the character stronger. There is nothing cool about the demons some people face on a daily basis. Don't.
Lastly, on a brighter note: write. Never stop. No matter what people say. No matter what criticism you receive. You will get better. You will have brilliant ideas. You will have something to share with the world. Never let anyone tell you otherwise. The ability to write worlds into being is yours to hold. Use it with pride.