I feel your pain! I myself have a bit larger bust size and found the sitting trot to be rather uncomfortable in that region when I don't have the bust support I need.
What I do is double up on bras, underneath goes the underwire bra that supports me just fine when not doing strenuous activities. Over top of that goes a fairly snug fitting sports bra that doesn't have any shaping, etc. basically, I strap those gals in! Lol. While two bras may not be the solution for everyone, it works for me.
ETA: SlideStop posted exactly my same solution while I was typing, lol.