Is the disease of obsessive thoughts (waswas), which comes in multiple forms and causes distress and depression, considered a type of disbelief (kufr), knowing that I am trying to draw closer to Allah and fear that I am being affected by these obsessive thoughts, and how can one be cured of it?

The treatment for the obsessive whisperings (waswas) concerning purity and prayer is to disregard them, and to consider that you have completed whatever you doubted. As for the obsessive whisperings concerning beliefs, they do not harm you, nor do you incur sin by them; rather, you will be rewarded for struggling against them, and your dislike for them is proof of the sincerity of your faith. You must seek refuge in Allah from the accursed Satan and say, "I believe in Allah and His messengers," and divert your thoughts to what benefits you.