How do I gain my parents trust back after losing it multiple times?

You sound like you want to change and i hope you can. talk to your parents. Really talk to them. Let them see you cry. Be real with them. It sounds to me like you might need some counseling to help you get over the hump and get your life back on track. If you show your parents that you really are trying this time, they might start to trust you again. It won't happen overnight and it will be hard work but it will be worth it in the long run.