I would say what you're doing "wrong" is not expanding your arena. I know jack shite about economics, but deductive reasoning and common sense? Yeah, I have those.
There are four means of income. Merch, TV, tickets, player sales.
You get a set amount for two of those every week along with ticket money which varies, but you can easily take your average ticket income over four games, multiply it by 11 (home games you have) and divide by 14 (weeks in the season) and come up with a weekly average for that too. I would suggest expanding your stadium so that with these three incomes you are breaking even, or very close to it whether it be +/-
Then player sales are used to upgrade your squad. You know, training young guys, selling them and your current starting ___insert position you don't train here___, then buying a better player for that position and using the rest on your arena.
Again, I see too many complaints about the short term and worries about the long term, instead of the other way around.
Come up with a three season plan, stick to it, adapt when you must, and complain freely when you lose or a guy gets injured.
I've said it many times here, proclaiming it even as my creed. Treat this game exactly like it was a real team in real life, and don't worry about changes made by its creators. Princeton grads who are astrophysicists generally know what they're doing when it comes to this stuff. (booze and women? that's another story, and an area best left for seedy, bearded, scotch drinkers like me to police)
You know the old saying? I'm a politician, when I'm not kissing babies, I'm stealing their lollipops.
Well, I was married to a molecular biologist, and scientists have their own saying: Without continual growth and progress, such words as improvement, achievement, and success have no meaning.
I want the site to improve, so I can achieve success :)
You should too.