As someone who works (but doesn't sell) healthcare and health insurance I strongly recommend finding a third-part insurance agent you can trust to help walk you through Medicare + Supplements. It all depends on what you actually are paying for vs what you THINK you are paying for.
Plan F, Plan G, Plan N, Medicare Advantage, PPO, HMO... etc etc. The name of the insurance (BC/BS, Aetna, United) means much less than the actual policy.
It can be so confusing and complex I wouldn't even try to recommend a plan or company to you on TexAgs. Find a reputable agent and block out some extended time to get educated.
Also, be sure the medical services you are using are actually billing your Blue Cross plan correctly. It could be that Blue Cross is a great plan for you, but the services are not being billed in the correct order (Medicare 1st then BC/BS or vice versa) and so everything is being denied.
Lastly, be aware that it is also possible for your providers to be billing for services that are not covered by Medicare (T-codes are a good example). If so then no one is going to pay for those but you.
I know it is all too complex and much too hidden, but you really have to make yourself sit down and learn. You are the only person that cares about your money. Not any of the insurances, hospitals, billing teams, etc.