If Allah "guides whom He wills and misguides whom He wills," then where is our own free will and determination?
Guiding and misguiding is in fact a consequence of people's choices. Allah gives people what they want out of His justice. If a person persists in believing, He increases their guidance. If they persist in the opposite, He increase them in what they've already chosen multiple times over. While guidance can be a pure gift for which we did nothing to deserve it, misguidance is always earned after many chances to accept the truth.
Regarding His saying, "Allah sealed their hearts," Ibn 'Abbas explained, "It is they who first kept closing their hearts, until Allah gave them what they insisted upon." His student said, "It is like a person closes his hand one finger at a time until no light reaches the palm. Only then does Allah seal it."