Is clothing purchased with money obtained by asking one's father for a larger sum than needed, for the purpose of using it for studies and sports, considered unlawful (haram), and must it be abandoned? And will not feeling remorse upon stopping this act prevent prayers from being accepted and lead to remaining in a state of sin?

If you deceive your father by taking extra money beyond the price of your clothes to spend on purposes he did not authorize, then this is a breach of trust. To achieve repentance, you must seek your father's forgiveness and return the excess money to him if you are able. As for the clothes, you are not obligated to return them. Not feeling regret at the beginning of repentance is a desolation and a "stain" left by sins, and one must continue to abstain, resolve, and seek the causes of regret. Whoever fulfills the conditions of repentance, their supplication is answered, and previous sins will not harm them.