Forgive us our trespasses if we are forgiving of others for theirs against us. Otherwise forget it. Generally God immediately forgives the sins of those who forgive others theirs and seek to know and do God's will in future. Sometimes reparation is called for if one's sin has defrauded others. .
A sin which harms no human being is probab;y not a sin against God, because God can't be harmed by mortal flesh. All that's left would be sins against one's self, and those are all foolishness which can be avoided by becoming wise. I find it difficult to think of any actual sin that is not sin against other. Even blasphemy is against God, though it's hard to imagine how blasphemy can actually harm God. Disbedience is a sin which may seem to potentially harm only ourselves, but Adam and Eve's disobedience, whatever that may have been, apparently harmed everybody else, (even the snake), besides themselves. .
In the Anglican view, I suppose technically zero (depending on your definition of 'prayer'). Formal forms of penitence are ceremonial actions, useful for us worldly creatures that find comfort in worldly ceremonies, but wholly unnecessary for salvation. What is required is a genuine, authentic repentance from the heart - the kind that comes intuitively from a regenerated spirit through faith in Jesus Christ. A prayer for forgiveness from someone truly unrepentant is empty. A prayer for forgiveness from someone truly repentant is superfluous. God has already heard the 'prayer' from within your heart at the point of repentance, the mechanics are for you not for God. All that said, I find praying for forgiveness is meaningful on a personal level for me. I've memorised the first half of Psalm 51 and find that general confession quite comforting. As well as the usual Anglican general confession: Merciful God, my maker and my judge, I have sinned against you in thought, word, and deed: I have not loved you with my whole heart; I have not loved my neighbour as myself; I repent, and am sorry for all my sins. Father, forgive me. Strengthen me to love and obey you in newness of life; through Jesus Christ our Lord. Amen Say it as often as brings you comfort. One time or a hundred times, do what brings you peace. If it doesn't bring you peace, find a different way - God loves sinners and saints, and God favours all with faith, no matter what prayer you say or when you say it. And when you find closure make sure you remember to pay it forward and forgive those who next transgress you, as God forgave you.
We do not think of devotion in this manner; it seems too mechanistic to us; one prayer with tears abides weeks of prayer that do not touch the heart that being said, for absolution you should either go for a public liturgy where the general absolution is said, or go for private confession to ease your conscience
Are you sorry, and is it your intention to avoid the sin in the future? Did you tell God you're sorry? If yes, you're forgiven. Go on, and show your love for God by trying to do what's right.
Just to add my voice to others here: all you need for forgiveness is a repentant heart. Say your prayers and go to church, not to be forgiven but to live in the light of Christ our Lord. No one is righteous before God; everybody sins. But through Christ our great advocate and intercessor, we are justified and saved. From Luke 8:49-50: Do not fear. Only believe.
You might benefit from the teachings of the Anglican Divines: Lancelot Andrewes, On the Power of the Keys, and Absolution (1600) https://www.anglican.net/works/lancelot-andrewes-on-the-power-of-the-keys-and-absolution-1600/
It is not how much you have to pray but are you contrite. You don't have to perform acts for the forgiveness but just throw yourself on Christ mercy. If you made a dump promise you can be forgiven of it and released from it. If you did anything you can be forgiven of it. Just trust in God's mercy