Frustrated with My JRT

Hello I'm hoping someone out there can help me. My JRT is one year ad d6 months old. Both my husband and I are AD military w/my husband currently deployed and me working 10 hrs/day. My JRT stays outside while I'm at work, but I have to put her in a small pen. I have a huge yard, but she always finds a way to get out. I'm afraid someone will steal her or run over her. I got so frustrated with her today, for she know she does wrong, because as soon as she knows she got caught outside, she runs back in and hides. Today for the first time I got so angry, I was about to hit her. I've never hit her but she cowered down and it broke my heart. I'm losing this battle and don't know what to do...help!