Product: Credit card

I had a XXXX credit card XXXX XXXX XXXX XXXX XXXX, when paying recently was given option to open new card with better interest by the shop check out person, applied and I got a second card. XXXX  years interest free was confirmed and I made purchases on both cards split costs and started payments over minimum due to clear accounts, XXXX card was set correctly by the merchant and interest never applied, this is now paid off, the other card, was set incorrectly by cashier and not applied the free interest, which since Citi told me would be XXXX  months not XXXX  years, regardless after XXXX  months a {$4000.00} balance had increased by approx {$1800.00} in interest which clearly wasnt what Id signed up for, I had a XXXX same month and hospitalized a week, thereafter was in recovery XXXX  months out of work, as Id lost my vision partially. As soon as I could I called to rectify, I was given a different story by each person I spoke with and no clarity, as to why this happened, XXXX I called back and asked they investigate, rectify and write to me to have this back on track, advising them Id be out of country for entire XXXX and to put a note on system to write, not call as I could not receive calls overseas. I was assured this would be resolved, when overseas I got twice daily calls to chase payment, they became very aggressive via phone and threatened to take action if I didnt pay, I could not call back so when I returned tried to resolve. Calling citi is impossible, they keep putting you through to same associates whom are robot like and refuse to help, I escalated, by then after many exams I was advised my XXXX had an issue and I needed XXXX XXXX, I was looking to move overseas as care is more affordable there, I told the XXXX this and that Id pay full balance if this was resolved immediately and incorrect charges reversed, she literally laughed and said its my problem. I called back, asked to escalated to another XXXX, but the associate was very helpful and said hed try to resolve it as this is a reoccurring problem and store inputs the wrong info but they can override it I was happy with this so he tried to resolve it, he could not as he said someone blocked it in the system so he escalated to a XXXX but I should not worry as it was a simple fix he put me through to the same horrific XXXX that was abusive before and she told me it is my problem to fix it and over XXXX passed so the fees stand, now how can a reputable business be this abusive of power? It is simply unacceptable, they also report this default to the credit agencies and brought my credit XXXX points down which impacts my ability to get any credit in future, they single handedly destroyed my history which was impeccable to date, to try and put their associated mistake in the consumer, refusing to accept and rectify their mistake is simply unacceptable and now they are sending me letters of demand to pay or get creditors to chase me as XXXX sell debt I will have this disputed and request they pay me for the stress and undue grief caused at a time that I had a XXXX and last week had XXXX XXXX, they are liable for this stress and I shall not be abused in this matter, I never refused to pay whats owed, but I refuse to pay outrageous fees, now {$4000.00} is almost {$10000.00} with their added interest on a balance that as of today should have accrued no interest as still in free period I signed up for. I have XXXX other store credit cards/ citi with XXXX balance, I am not a bad payer or customer and they need to immediately rectify this and rectify my credit as well, thank you