When I was almost 19,I committed a terrible sin and I regret it a lot,and I repented from this sin and didn’t return to it again..but Iam afraid that ALLAH will punish me for the sin I committed because its considered from the senior ones..and if the repentance was sincere from my heart will the sin stay with me...returning to the famous quote : As you sow, so will you reap?..

My brother,as long as you repented sincerely and regretted and promised ALLAH that you wont return to the sin and did a lot of good deeds then your repent is accepted if ALLAH wills,because he promised us,and he doesn’t renege his promise,ALLAH almighty said ( ألم يعلموا ان الله هو  يقبل التوبة عن عباده..) And my brother,even if your sins reach till the sky then you repent from them ALLAH will forgive you and he loves you, ( ان الله يحب التوابين)

And you have to avoid everything that reminds you of the sin and the sin wont stay with you and the quote doesn’t apply on you as long as you sincerely repented.