Am I to be considered a disbeliever for what happened in my case, taking into account the opinion of those who deem one who abandons prayer to be a disbeliever?

You have reached an extreme level of waswasah (obsessive whisperings). Doubts after completing ablution or prayer should be disregarded, especially with waswasah. One who considers the deliberate abandonment of prayer to be disbelief does not include someone who has prayed and wishes to repeat it out of caution, or someone preoccupied with a condition of prayer. Know that you did not intentionally abandon the prayer; rather, you thought that the time was still remaining. So, turn away from these whisperings. It is obligatory for you to act upon the fatwa of a scholar, and not to issue a fatwa for yourself. We ask Allah to grant you well-being, and we advise you to constantly supplicate and to consult a trustworthy psychiatrist.