There's nothing unusual about your attraction to someone other than your wife, and sexual fantasies are perfectly normal. We all have them; it's part of being human. Stop worrying about your fantasies and enjoy them. Sometimes your wife is thinking about you when you're banging her, but sometimes its Ben Affleck or whoever gets her juicy. That's perfectly normal too, and you both enjoy the sex either way.
Your attraction to Jen isn't the problem, and I don't think your fantasizing about her is either. You're just afraid, and fear is uncomfortable. You know that if you got caught cheating, it'd fuck your life up pretty badly and you don't want that. Fair enough. You've got a couple of options; pick what works for you.
First, you can try disciplining yourself to stop thinking about Jen. Stop watching her, stop fantasizing about her. When you become aware that you're thinking about her, think about everyone in your office finding out. Think about your wife, how hurt she'd be, how angry she'd be, how betrayed she'd feel. Think about how stupid you'd feel for throwing away your marriage, losing half your assets in a divorce, just to bang the office hottie. Think about Jen, disgusted with you for hitting on her when you're already married, turning you down, laughing at you and telling everyone what a perv you are. Basically, associate sexual thoughts about Jen with strongly negative feelings.
Or, maybe behavioral modification and psychological bullshit isn't your thing. You could try honesty. Talk to your wife about the hot girl in the office, tell her you think about the girl sometimes, reassure her that you will not cheat on her, that it's just an arousing little fantasy to play out in your mind. Ask her about her fantasies. Tell her about others you've had. She might have some awesome little ideas that could be fun to try out. You can explore and fulfill each other's fantasies safely with each other, adding spice to your marriage, and nobody gets hurt. Roleplay. She can be Jen for you and you can be Rodrigo the lawn guy for her, if that's what she's into. It's all good clean fun and based on honesty and intimacy and trust.
It's a carrot-and-stick approach, and really only the carrot should be necessary. If taking your wife to your office some Saturday night, banging her in the back seat of your car in the parking lot, calling her Jen while you cum doesn't satisfy you, then its time to apply some of the stick and focus on other kinky things that you can do with your wife.
Good luck, anon.
