If you work your chest more you'll tighten those muscles more which will give you rounded shoulders and make the situation worse.

It's difficult without seeing you but, as Kel says, improved posture. Make it a habit in you everyday life to retract your shoulder blades and keep your shoulders back. Most of it is habit but specifically training your rhomboids and middle traps will help.

Make sure you stretch your chest after training too to try and loosen it up. If its tight look into PNF stretching.