Does the prayer of one who intended to change the formula of the Tashahhud after starting with the first formula, then abandoned the change for fear of invalidating the prayer, become void, knowing that his intention accompanied the beginning of reciting the Tashahhud with the first formula?

Your prayer is valid, and nothing more is required of you. Whichever tashahhud you recited, your prayer is valid. There is a disagreement among jurists regarding the final tashahhud in a fard prayer: some consider it a pillar (rukn), while others consider it a sunnah. Be careful not to open a door to waswasah (whispers of the Satan/obsessive thoughts) for yourself.