I know it's easy for me to say and not exactly what you want to hear, but your dentist was correct. You have left an active infection in your jaw which has been destroying vital bone which supports that tooth, the adjacent teeth and potentially other important structures.
Your delay in treatment may have made a bad situation worse. If you cannot afford to save the tooth then extraction may be your last resort. It is better to remove the tooth than to allow the infection to continue. The best option would be to save the tooth but it is understandable if that option is not viable for you. If you live near a dental school or a hospital that has dental residents, you may be able to have the root canal done at a reduced fee.
Information contained within this reply is intended solely for general educational purposes and is not intended nor implied to be a medical diagnosis or treatment recommendation. This is not a substitute for professional medical advice relative to your specific medical condition or question. Always seek the advice of your own doctor for medical condition. Only your doctor can provide specific diagnoses and therapies