Are the obsessive thoughts related to shirk, which cause fear of the non-acceptance of repentance, considered shirk, despite friends' reassurances that they are mere whispers (waswas)?

If you are experiencing obsessive thoughts (waswasah), you must distance yourself from indulging in them and completely avert your mind from dwelling on them. Occupy yourself with learning, righteous deeds, remembrance of Allah (dhikr), recitation of the Quran, seeking forgiveness (istighfar), and supplication (du'a). Shirk (associating partners with Allah) is not a trivial matter. Whoever embraces Islam with certainty does not exit it except with certainty; there is no room for probabilities in it. The Prophet (peace be upon him) guided us to a supplication for seeking refuge from shirk: "O Allah, we seek refuge in You from knowingly associating anything with You, and we seek Your forgiveness for what we do not know."