I'm sorry—I've felt some of the feelings you're experiencing, and I know it must be tough for you. In my mind, it seems that you're struggling with two ways of thinking, so I'm going to address them separately.
First, you regret the negative effects your actions have had on others. This must hurt because you feel that your friends' current unfortunate conditions are a result of your actions. I hope you realize that, regardless of your influence, your friends' choices were their own. I would also suggest doing everything you can to make it right—apologize to them, try to be a good influence on them, and ask for God's forgiveness. As you do what you can and realize that the mercy of Jesus Christ can both pay for your damage and help your friend, you will be able to forgive yourself. It may take a while, and you will likely always regret your negative actions, but you can find peace even if you are unable to undo the damage.
Second, you feel like the good influence you have is small or even negligible. This concern reminds me a lot of missionaries wondering if they're making a difference, so I'm going to paraphrase Preach My Gospel:
Your success as a [human] is measured primarily by your commitment to [help] people....
Avoid comparing yourself to other [people] and measuring the outward results of your efforts against theirs. Remember that people have agency to choose whether to [make good choices]. Your responsibility is to [help] clearly and powerfully so they can make a correct choice. Some may not accept your [influence] even when they have received a spiritual witness that it is true. You will be saddened because you love them and desire their [happiness]. You should not, however, become discouraged; discouragement will weaken your faith. If you lower your expectations, your effectiveness will decrease, your desire will weaken, and you will have greater difficulty following the Spirit....
When you have done your very best, you may still experience disappointments, but you will not be disappointed in yourself. You can feel certain that the Lord is pleased when you feel the Spirit working through you.
And more, from here:
When people choose not to [make good choices because of your influence], your work is not wasted. Your consistent efforts in serving and teaching as many people as you can is one way God prepares His children to eventually receive His [help]. He often reaches out to His children through you. Even when people do not accept the opportunity to [make good decisions], your service and words are evidence of God’s love for them and may plant seeds that future [friends] will harvest.
As these quotes say, you can feel good about yourself if you're doing what you can. Are you the most effective tool to help your friends? Probably not, but that's because few mortals would be. But God doesn't just want to help your friends; He wants to help you too. And if you can become a better friend and person by trying to help, even if your help isn't immediately successful, you can feel like a success. You can know that God is proud of you.
I hope that helps; please email if you want to talk some more.