If the alternative is not exercising then it is probably assur not to listen.
Side point: you say “it is difficult”. Many things in life are difficult. Lifum tzara agra. Hashem put us on this world to pass nisyonos. So just because something is difficult shouldn’t be a reason to go “looking for a heter”. Kol kula tzricha bdika. Take a step back and think about it before jumping to heterim.
When it comes to this particular question of music during sefira / 3 weeks in general (not for exercise), this always bothers me. I enjoy music, but I won’t die without it. I realize that chazal instituted a period of aveilus and it should be taken seriously. Think about the “spirit of the law”, rather than “the letter of the law”. It is probably better to listen to a slow, hertzidig D’veikus song then some of the acapela nowadays. I think the best thing is to listen to those type of songs without music or with only acapella music.
Of course, if you are lisheim shamayim and you really feel that you can’t exercise as well as you should without it, then by all means, find a heter. (Also, if you feel you need it to keep you awake while driving then of course you should listen to music)