You've met some terrible people
Every writer throws part of themselves into their characters but it's also useful to have a hate bank of people who have wronged you. Just cleverly rename them and throw them into your novels and make the rest of the world hate them too.
Joanna Bolouri
Your bullshit tolerance is low
If it's easier to wade through all the nonsense in your own life, it should be easier to edit all the nonsense which is undoubtedly filling the pages of your first draft.
You don't feel the need to please everyone
By now you might be at the stage in your life where you don't give a rat's ass what other people think. THEY ARE NOT THE BOSS OF YOU. Your work will reflect this. Your opinions are yours and you don't care what anyone else thinks. Unless it's your editor. You should care what they think. (love you Kathryn)
You've lived a bit
You have experience. You know shit that you didn't know when you were in your twenties. Maybe you've travelled. Maybe you've had 17 kids and now wish you had 17 cats instead. Whatever you've done up to this point, use what you've learned in your writing.
You know what makes you laugh
When you're writing comedy, it's important that you find what you're writing funny. If it doesn't make you laugh, it doesn't go in.
Nostalgia can be useful
Recreating events you remember fondly can be a joy to write and a pleasure to read. Set something in 80s when you were a kid and everything was simpler. Listen to terrible boybands from the 90s while you work and you'll be surprised how they invoke not only feels of rage but also fond memories.
You've had your heart broken
You can create some of your best (and worst) work while you're wailing and beating yourself up over someone who broke your heart. Also, in time you will wish nothing but bad things for them and this can also prove useful on the page. Being able to write how broken you are is very cathartic.
You know how to do sex
You've done it at least twice by now so this makes you an expert, handy when writing sex scenes, especially bad ones. You are now the Queen of Dirty and your work should reflect this. Unless you're writing for kids…
You have enough embarrassing material to last a lifetime
All that stuff that keeps you awake at 2am and makes you wish you were dead is all invaluable. It's quite cathartic writing a character who makes all the mistakes you have, without having to admit that it's you.
You know yourself.
This is more about feeling confident within yourself than some sort of existential awareness. Knowing your strengths are your weaknesses are invaluable assets for any writer to have. I would love to write horror but I know my strengths lie elsewhere.