Perhaps the most frustrating thing about witnessing is the fear of doing it wrong. If I say the wrong thing, if I don't act in a Godly manner, perhaps even if I don't explain it right, they might reject it.
But the thing about sharing the Gospel is that while they might reject it if I tell them, they can't accept it if I don't. God's command to us is to just tell them. If they reject it, it's on them. If we don't tell them, their sin is still on them, but we would share responsibility because we didn't even have the guts to tell them when we could have.
As Christians, we can be grateful that no sin we have ever committed, or will ever commit, will be remembered in Heaven. If we fail to share the Gospel with someone we could share it with, that's on the cross. But if we don't share the Gospel with them, theirs won't be. You can't believe what you've never heard, and since you need faith to be saved, they can't be saved unless they hear. Therefore, we should be as willing to witness as we are to call an ambulance when we find someone bleeding out on the streets. Let no man's blood be on your hands. Preach, and don't let your fear dictate someone else's fate.