I once wore a strapless bra to a family wedding under my favourite halter dress. I felt fabulous, like movie-star fabulous. After a champagne or three, I jumped with gusto to catch the bride's bouquet and in the blink of eye (or in my case, the click of a billion embarrassing photos) my strapless bra flipped down over my stomach.
You may have had some good times together, and it's supported you when you really needed it. Although it can be tempting to hold on to your favourite bras, especially the old-faithfuls which are oh-so-comfortable and easy to wear, you need to break up with your sports bra when it doesn't support you as well as it used to.