Very poor customer service by comcast - Issue with billing and comcast wrongly sent to collection agency

I am not happy with comcast service at all, have been working with them for 21 days to resolve a billing issue. No one wants to help and no one cares. Chat is not at all help, they accept the mistake by compacts and just create a case, but nothing happens.  How can I get help with this? have been with Comcast for 20+ years and I am sure they are going wrong way when it comes to customer service. Story goes like this - comcast sold me 1 GB plan and made me switch and created a new account  assuring that I will not be charged any amount. Customer rep did not connect old account with new account and hence triggered a early termination fee and they did not notify me. I heard form collection agency, when chatted with comcast on 5/31 they said they have a case and will help in 2 weeks. When followed up today they said yes the link between new and old account is missing and they created another case number ending with 33419 today for resolution. How many more days I have to spend with comcast to not impact my credit score? This is not at all acceptable. Any help ?