Is it considered disbelief to believe that the "day" mentioned in the verse: (Indeed, your Lord is Allah, who created the heavens and the earth in six days) differs from our concept of a day, and that it is not like the verse explained by the Prophet, peace and blessings be upon him, which says: "Whoever interprets the Qur’an according to his opinion, let him prepare his seat in the Fire"?

It is clear that you are suffering from whisperings (waswasah) concerning matters of disbelief (kufr) and mockery due to your indulgence in them. The cure for this is to turn away from them and not pay them any mind. So, be at ease; you have not disbelieved by what you imagined. A group of exegetes (mufassirun) have stated that the days of the creation of the heavens and the earth are not like our days, and that they could be a thousand years. Allah created the heavens and the earth in six days for wisdoms, among them teaching His servants gentleness and deliberation.