Misled into larger payment

I ordered a monitor about 18 months ago- one that came along with a 3 year warranty. Afterwards, I purchased a new modem, and was shocked to discover that two years had been taken off my warranty. The sales executive refused to reinstall my previous three-year warranty, even though I had purchased both the monitor and modem separately and not in a set. Their online ad had explained that a 19' inch monitor comes with a three year warranty, and I was denied from this claim.

Recently, I called the sales agent and asked whether I am required to pay in full, before the last 18- month payment. I was told to continue my regular payments, as I did, and was surprised to find that my next bill was extremely high. I am unable to pay this amount and it is not fair. I feel that I have been fooled, cheated by their business! I would like only to pay my original balance- the one that I am supposed to pay.