I'm struggling to find a good work-life balance.  By the time I get home at night I'm too exhausted to interact much with my girlfriend at home
I know where you are coming from. Why do you think you're exhausted?
I work 10 hours a day.  I get up before work to go to the gym, so I'm extra tired by the end of the day.  My job unfortunately doesn't offer flextime
10 hours a day it's a lot. Usually shifts are 8 hours, doesn't your company give you an extra day off work to compensate you for those many hours?
I'm salary, so even though I'm only technically working 40 hours a week, I need more time to get all of my work done.  Since I'm salary, I get no overtime even though I'm putting in an extra 20 hours each week
I'm really sorry to hear that. Another cause for exhaustion is dehydration. Do you drink enough water?
Yes, I keep a reusable water bottle with me at all times.  I drink at least a gallon every day.  It's the bottle I always take hiking with me.  Hiking is the one thing I do on the weekends to help maintain the work-life balance.  
What do you most enjoy about hiking? Maybe other activities you could do after work would help as well.