Is seeking forgiveness and repentance sufficient if someone says: "I cannot even pray as I should," believing that his intention was not to diminish the importance of prayer?

There is no harm in what you said; your phrase does not diminish the importance of prayer. Rather, it is understood that you mean you are unable to perform the prayer as it should be performed, and prayer is the pillar of the religion and the first thing a person will be held accountable for. Therefore, abandon obsessive whispering (waswasah) because it can lead to great evil.