What should i do?

the bank treated me very badly, and i wrote a letter to them to complain. They completly ignored me so i sent my letter further up the company saying i had already contacted my local bank with no response. I then got a phone call from my local bank (presumably because i had complained further up) and since i missed the call they left me a voicemail asking me to call them back. What they did was deffinatly wrong, so i want to be firm and see if i can get any compensation for it. However, when i speak to people in person im always really weak and usually cave. I cant stay angry and always end up saying it doesnt matter. Is there anyway i can convince the bank to do this simply over letter as i had written to them or is there any suggestions on how i can stay firm on the phone?