How can one get rid of the obsessive thought of interrupting prayer, which recurs despite there being no intention to interrupt, and is the prayer considered valid in this case?

You must turn away from the whisperings and pay no attention to them, for indulging them will cause you distress. Strive against yourself and do not surrender to the devil's enticements. Beware of interrupting your prayer because of these whisperings. Whenever your soul tells you to cut off your prayer, repel this thought and continue with your prayer. Strive to supplicate and seek help from Allah Almighty.