Here's the biggest secret to why your friends' advice or your own attempts to correct your speech has not worked: your natural brain processing has been working against you.
Once you learnt how to talk at a young age, it is your natural brain processing that you start to do it all subconsciously. Your brain is designed such that you are not aware of the physical aspect of talking. You can walk and talk. You can drive and talk. It's like walking.
You learn to use the necessary muscles without knowing exactly how they work. You become like the technically proficient ice-skaters that now focus on 'artistic presentation'.
It doesn't help that statistically only 1% of the population stutters.
So just like the 99% of your friends, when you think about talking, you are more likely concerned with what you say, what words do you choose to achieve your purpose (be persuasive, to not hurt someone's feeling etc), or how to, as your friends may suggest, "sound confident".
It is natural that you are not aware of how your brain controls your speech muscles in order to produce all the hundreds or thousands of different words you say everyday. And that 'neuro-motor control' or 'speech motor control' is exactly what all the research point to as the issue in stuttering.
In order to get to the root cause of stuttering, you need to address exactly how you produce speech. You need to 'de-construct' exactly how you talk and learn the speech 'technique' to talk fluently.
You have seen this in action. A footballer or a baseball player may look like s/he is 'kicking or hitting a ball' just like anyone else, but he actually kicks or hits in a very specific way that achieves a spectacular result, such as curving the ball in order to beat his opponent.
In the same way too, it is possible for you to learn to speak in a different way that actually makes it virtually impossible for you to stutter. I call this your speech fluency 'technique', and see this as a 'tool' for you to apply in order to solve your problem of stuttering.