I know the issue you are talking about! A lot of my friends who workout a lot really struggle finding shirts that fit. I have only been working out for 2 years now but I also have the issue.
I know you said you don't want to change brands but I have found a brand that designs shirts specifically for the athletic physique. Basically they complement the broad chest/shoulders and taper down to the narrow waist. The brand is called Tailored Athlete, you can find them here - www.bodybuildingclothing.org.
Let me know what you think!
All the best,