the suggestions above may work, but when i'm working with athletes it's pretty clear it's not sufficient to prescribe a suite of motions until the athlete's been checked.
In your case it may be that the lower trap isn't firing well, and even if you do those exercises, you'll still only be partially engaging that muscle.
It may be that you always carry your bag on that side so you've conditioned yourself to hike up your shoulder.
it may be that the movement in your opposite side hip is jacked a bit and you're carrying the compensation in your shoulder. so mobility work may be the best course.
In other words the site of the issue mayn't be the whole issue, and without addressing the whole issue the speed of resolution can get slowed down.
That means, if you're keen to get this addressed quickly and effectively, see a movement specialist who can do an assessment and help you regain your body.