Why not share the responsibility? If there are particular tasks which are more onerous, or particular times of day when they are more challenging, why not bring in a professional caregiver for those jobs? This way you may be able to give more of yourself when you are caring for them.
Why not focus on enjoyable times with your parents? It is not selfish to want to spend quality time with your mom or dad, and to delegate some of the other jobs. Sometimes it might be more relaxing and less stressful for everyone, if a professional caregiver takes responsibility for some jobs such as bathing, housekeeping and toileting, and you spend time talking, laughing or going to gentle walks together.
Why not take a break every few months? There are different respite options, including bringing in a caregiver to their home, or a short-term stay in a retirement or supported living home. Plan these into your routine and you are less likely to suffer from exhaustion and burn out.