V.B.
I never learned or enjoying trying to learn to run until about 5 years ago. That's when I bought the correct sports bra for me and it changed everything. You might be different and you might be disbelieving, but I found the best ones (for me, as a 34/36 C/D/DD, depending on weight fluctuations) to be a cheap Champion bra from Target.
The problem I'd had was I was trying to wear compression sports bras. Compression bras do exactly that... compress everything, and it makes breathing more difficult. Don't buy compression bras. Just get a supportive sports bra. The Large Champion ones with the wider straps work fine for me. The ones with the strappy straps do cut in a little bit and I can only wear them when I am on the smaller end of my weight fluctuations.