Taubah and Repentance

Q: Should one ask forgiveness for a sin if one is unsure whether one has committed the sin at all?

Q: If someone repented from a sin, and then in the future was thinking about the sin they committed and took pleasure in the enjoyment if the sin they once committed, do they have to repent once again?

Q: I heard that if someone does zina and asks Allah Ta'ala for forgiveness, Allah Ta'ala forgives that person but that person wouldn’t be allowed to see Allah Ta'ala in Jannah? Is it authentic?

Q: I have committed sins that I am not proud of that nobody knows. I try to forget them but I can't. What should I do?

Q: If we had committed any sin in the past and we are not sure if it was a sin, should we ask any Mufti or Aalim regarding that thing or just repent from all sins and move on in life believing Allah has forgiven us?

Q: Please can you tell me how do I make sincere repentance for the sins major/minor that I have committed in my life.

Q:

1. Is saying Astaghfirullah throughout one’s life fardh?

2. If a person repented without saying Astaghfirullah what is the consequence?

Q: Is it permissible for a person to reveal his past sins in the following situations:

1. With the intention of others taking lesson from it?

2. In the case where the sin is such that the one being spoken to is not involved in that sin, nor capable of doing that sin, and one reveals one's sin to him, to show him how one had changed one's life?

Q: How do I forget my past sins? They haunt me everyday.

Q: If a person recites this dua thrice أَسْتَغْفِرُ اللهَ الَّذِي لاَ اِلٰهَ اِلّٰا هُوَ الْحَيُّ الْقَيُّومُ وَأَتُوبُ إِلَيْهِ , Allah Ta'ala forgives all his sins. Does this forgiveness include major sins or is it only for minor sins?