Bad shoulder posture (rounded /slouched)

I have a bad posture when it comes to my shoulders. They tend to slouch forward and look rounded. The problem has been there for almost 5 years since I was in school (probably because we had to carry heavy bags in 10th standard). I have been working out for a while, and I have lost a bit of fat and gained some muscle and strength. However the posture has not improved. I also get pain in my shoulders sometimes, which is quite random. Sometimes I get it after I sleep during the night. Sometimes It comes when I do some exercises (like chest Flys). I would like to know who I should consult, and what could be possible solutions. I've read that there are certain belts or vests, which help improve posture. Do these help? Thank you.