Well for starters you've played since season 5 (quite impressive, not a lot of people can say they've stuck around that long. I've played since season 2) and only amassed an arena the size of 7700. Invest heavily in the draft and expand that arena. The bigger the arena, the more revenue you can use to have a better team. The better team you have, obviously the more you'll win games. If you invest in the draft and land a stud, he could change the entire landscape of your team.
I've started over several times now, each time because my arena was never big enough to support the salaries I had acquired. So I trained and trained, then sold shop and spent most of the money on expanding arena and buying 3 new trainees and starting over. It takes some time but eventually you'll get to where you can buy the players you need to promote and afford it.