Humans are imperfect.. don't worry. Yes, you may've committed a major sin, but don't worry. Allah [swt] mentions in al-Qur'an al-kareem that he is forgiving, compassionate, merciful. However, not all seems to be negative for you at the moment, masha'Allah you have mentioned you pray and read Qur'an, and that's great masha'Allah. A tip also.. don't reminisce to the extent that it affects to your well-being in a negative way insha'Allah..
You must never give up on repentance: turn to Allah [swt] for your sins, and repentance consists of the following:
* Promising you must never commit the sin again, no matter what!
* Feel a deep regret for the sin, and realise that what you did is wrong
* Start to 'repel evil with good', i.e. start commiting good deeds from now on!
Remember, Allah [swt] is all merciful:
...For He [Allah (swt)] is Oft- Returning, Most Merciful.
2:54
Except those who repent and make amends and openly declare (the Truth): To them I turn; for I am Oft-returning, Most Merciful.
2:160
...For verily Allah is Oft-Forgiving, Most Merciful.
3:89
Allah accept the repentance of those who do evil in ignorance and repent soon afterwards; to them will Allah turn in mercy: For Allah is full of knowledge and wisdom.
4:17
And remember; we humans are prone to make errors, what you must do, though, is to learn from your mistakes, insha'Allah!
& Also:
Allah the Almighty has said:
"O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind.
O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you.
O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to face Me, ascribing no partner to Me, I would bring you forgiveness nearly as great as its."
[Related by Al-Tirmithi].
Take care!