What is the ruling on the prayer of someone who has many doubts, and consequently acts on the greater number, and is unable to concentrate on every word he says, such that if his mind wanders during one of the transitional takbeers, he considers that he did not say it?

If doubt has reached the level of obsessive whispers (waswas), it should be disregarded, and one should build upon the most certain outcome. What is doubted should not be acted upon, and prostration of forgetfulness (sujud al-sahw) is not required. Your prayer is valid in this case, and lack of concentration in prayer has other fatwas.