Is saying ‘astaghfirullah’ with my tongue enough, or do I have to feel something in my heart for it to count?
Both matter, but not in the same way. Saying astaghfirullah with your tongue is never wasted, it is a blessed habit, and the more your lips move with it, the more your heart is drawn to soften. But you have sensed something true: the living heart of tawbah is not the sound, it is the small ache of regret within. The Prophet, peace be upon him, put it in a single phrase.
“Remorse is repentance.”
The Prophet (peace be upon him), Sunan Ibn Majah (4252) · meaning
He did not ask for a ceremony, or for perfect tears. That quiet inward tug, that soft “I wish I had not,” is the seed of return. The tongue opens the door, and the heart’s remorse walks through it. When both move together, even for a moment, the turning is real.
“The repentance Allah accepts is only for those who do wrong in ignorance, then turn back soon after.”
Qur’an, An-Nisa 4:17 · meaning
Notice the wording. He calls our slips ignorance, the momentary forgetting of a heart that still loves Him, and He asks only that we turn back soon, without letting the days pile up in delay. There is mercy in that word soon, it means He does not want you carrying the weight an hour longer than you must.
And if you say the words while the feeling has not yet arrived, keep saying them. The tongue is a path to the heart, and a habit repeated with sincerity slowly awakens the very remorse you were looking for.
One small step: The next time astaghfirullah passes your lips, pause on the third one and let your heart quietly whisper a single, honest “I am sorry” behind it.