Most of the time it's not as simply as fixing X. Most of the time X isn't the root cause but X is what's visible to the other person.

or they’ll just tell you that you need to be more confident

That doesn't happen to be bad advice. In reality a lot of extroverted people aren't socially popular because they ask other people for feedback and then integrate the feedback.

If you could implement the advice of "be more confident", your social interactions would go better. On the other hand it's seldom that simple.

1) “Hey, I’ve noticed you tend to do X.”

2) “It’s been bugging me for a while, though I’m not really sure why. It’s possible other people think X is bad as well, you should ask them about it.”

3) Paragon option: “Maybe you could do Y instead? I dunno, just think about it.”

4) Renegade option: “From now on I will slap you every time you do X, in order to help you stop being retarded about X.”

I don't think that's a good script. After saying “Hey, I’ve noticed you tend to do X.” the next step would be to ask a question like "Are you aware that you do X?" If you just assume they don't know that they are doing X your advice isn't targeted.

If you want to give helpful advice, ask a lot of questions to really understand the situation of the other person. It's much easier in a live interaction than online where there's time lack between responses.