Is saying, "I recite Al-Fatiha slowly so that the prayer is not invalidated," considered pessimism? And is doubting the purity of something considered pessimism? And does ablution become invalid if it is mixed with ostentation or pessimism?

What you have mentioned is not a bad omen or a doubt about something being impure. Rather, build upon the principle of purity and act upon it until you attain certainty of impurity. Read (the Quran) normally and repel these obsessive whispers, for there is no cure for them except repelling and disregarding them.