Can a Muslim be afflicted with sin despite their commitment to worship, and is this considered a seal upon their heart despite their struggle to abandon it and their return to it after repentance?

It is possible for a Muslim to be afflicted with committing sins while still persisting in acts of obedience, but he must strive against himself to abandon sin and not surrender to it. He must also review himself in the performance of his duties, as neglecting them may be a reason for committing sins, as Allah Almighty said: "So We caused among them animosity and hatred until the Day of Resurrection because they had forgotten a portion of that by which they were reminded."