"Megan Leavey" is a real person, a former Marine and Military Police K9 dog handler. In the movie, she's played by Kate Mara. Leavey is unhappy with her life, and decides to join the military. Once there, she sets her sights on the munitions-sniffing dog program, and trains ferociously to get in. She ends up with Rex, a particularly vicious German Shepherd that she manages to bond with. They're both injured in combat, and after she leaves the military she works to adopt Rex - although he's been listed as unadoptable.
I suspect they may have overplayed Leavey's malaise - both before she joined the military and after she left it, but I don't think they overplayed her determination: you don't get where she got without a great deal of strength of will. An interesting and enjoyable character study.