So I've been having a crisis of fatih. I truly wonder if Christ can forgive me for the things I've done. I've made so many mistakes, and I've caused so much harm. Is it really true that Christ can forgive me? How do I know that my sins are forgivable? Is sinceritry enough? I have so many bad thoughts in my mind...

I've stolen from my parents, I've engaged in drug use. I havn't stolen anything in years but I cant help but think about my sins. I've admitted them to my parents, and I now have a good relationship with them (my parents) Why do I still feel guilty?

...all your sins , past , present , and future are forgiven in Christ . Perhaps it is time to have a sit down discussion with your pastor .

God would forgive all of that in an instant my friend. I'm not perfect either and I've done everything that you've mentioned.. I'm not proud of it but you can't let it hold you back.
You have to forgive yourself in order to move on.

Jesus died once and for all. You are not measured by how many bad things you have done which determines your fate. Just remember the criminal on the cross next to Jesus who asked him to remember him when he died? Jesus told him at his death bed that he would be with in him paradise. Now what do you think the life of a criminal would be like and repent a few hours before death and end up in paradise?

If he can be forgiven for a lifetime of crime and such bad sin then so can you! You have even said that you haven't stolen anything in years.

Luke 23: 40-43 NIV

40 But the other criminal rebuked him. “Don’t you fear God,” he said, “since you are under the same sentence? 41 We are punished justly, for we are getting what our deeds deserve. But this man has done nothing wrong.”

42 Then he said, “Jesus, remember me when you come into your kingdom.[a]”

43 Jesus answered him, “Truly I tell you, today you will be with me in paradise.”

http://www.biblegateway.com/passage/?search=Luke+23%3A40-43&version=NIV