If you started in a country that was a C3 you need to understand that C3's operate from different rules (according to Wikia-simcountry). You probably have a few corps that are beyond hope. Wiki says to avoid basic industries and weapons corps (as I recall--my memory is definitely suspect). I read somewhere else on Wiki that Enterprises are inherently more profitable than Presidents when operating corps. I found that to be the case when I cut the corporate tax to 5% and invited CEO's to buy my corps and build corps. The idea is that a president can actually earn more from his taxes on workers than he gives up in corporate taxes.
Another thing that will help is to cut your corporate profit sharing. That also helped my corp values. I'd suggest you go to simcountry wikia, sign up and study all the hints they have there. Some of them are questionable, so you'll still have to experiment and use your own judgement.