Practice. Practice. Practice.
My prof set up a small reading group when I was in college. 5-6 students would get together every week and discuss a preassigned paper. I was like you, lacking in confidence, hesitant to disagree, handling confrontation badly etc. But the group created an environment where I could learn how to deal with these things. I could see others fumbling about and realized it was not just me struggling. When I got things right others encouraged me. All this slowly translated into increased confidence.
So try and set something up like that with friends/people who want to see you improve. Every week meet up and practice.
Also recommend Marshall Rosenburg's Non-violent communication. It really helped me see certain things I was doing wrong and showed me ways to avoid them.