Feeling Stressed

So I haven't been exercising much lately. Mostly because my job has me super stressed. There's this girl who works there (just a regular employee) she's full time but is never happy about anything anyone does (despite the managers and owners being happy) so she intentionally sabotages the shift after her (which is my shift) leaving us with more and more work to do (she's openly admitted to sabotaging the shift because she wasn't happy.) So anyways, this is the source of my stress and come the end of the night I can't work up the strength to hit the gym. I'm mentally exhausted at that point and just want to go home and sleep. Tried talking to the owner and managers about the problem but they're in this situation where they can't talk to her because if they do she'll quit (she's done it in the past and left for 6 months.) They can't afford to have her quit because we are really short staffed and are in the process of doing a load of hiring. So I'm stuck with the stress for the time being and was wondering if anyone had any tips to motivate me to go to the gym. After work is basically the only free time I have to hit the gym before work doesn't work for me because I have other responsibilities during the day once I wake up.