What is the ruling on memory weakness and the inability to remember the number of rak'ahs in prayer, or what was recited in it, or to concentrate during ablution, or when voices are loud?

The best solution to the problem of lack of concentration in prayer is to perform it in congregation, so that you are safe from doubt regarding the number of rak'ahs, and the Fatihah is not obligatory upon you according to many scholars. If you pray alone, try to concentrate and do not pay attention to doubt thereafter. If one experiences frequent doubt in prayer, they should ignore it, and there is no need for rectification, but they should perform the two prostrations of forgetfulness (sajdat as-sahw) after the salutation. Similarly, if one experiences frequent doubt regarding wudu'—whether they have nullified it or not—their wudu' is not invalidated, due to the hardship involved.