i'm not sure if I understand exactly what you're doing in that pen...but i'll tell you what I would do if this mare was dropped off at my place tomorrow - but first I want to say, it's NOT a bad thing that a horse might want to be close to you. As long as she isn't being pushy or trying to "take" your space, then having a horse want to be close to you is a very good thing. I would NEVER allow a horse to push into me or move me around, but I don't go with the "arm's length" or "X amount of feet" rules, I want my horse to enjoy being close to me, I want them to understand that when they are near me things are ok. If you see a horse close to a person, and the horse has it's head down it shows they aren't going to worry about that scary rock or dog or car because they know you are taking care of things. That's a horse that trusts it's herd leader (you). I actually do exercises with my horses to teach them to lower their heads and relax around me.
What I would do with this horse is put her in the pen with no lead rope or lunge line. Turn them loose and then you just stand in the middle of the pen in a very unassuming, non-threatening way and just look off into the distance and watch the horse out of the corner of your eye. As soon as the horse turns away from you or stops paying attention to you, shuffle and stamp your feet to spook her - so she looks at you. When she looks at you, immediately go back to your original unassuming posture. If she walks up to you, that's fine, you can pet her and reassure her. But keep doing this until the horse realizes that when she's focused on you, nothing scary happens, but when she turns from you or isn't focused on you, something scares her. I have done this with horses and eventually walked out of the pen and had them watch me for 5 minutes or so, while I was outside of the pen. I've had people criticize this because it sounds too simple to accomplish anything, but believe me, it teaches the horse to pay attention to you, it generally is a very very positive exercise and you probably only need to do it once or twice.
After you have gotten this horse's focus and started making it understand that it needs to pay attention to you, then I would start to lunge her. I have to admit, I don't have alot of experience with horses that won't lunge because i've never had the problem, but I think that you should also consider finding someone to work with you - I know you said you can't find anybody, but i'm sure you could if you look around. One person to keep them moving forward and the other to hold the lunge line is probably what this situation requires. If you have to do it yourself, I would just recommend having a long whip and if she won't move when you ask her to, really pop her rump with it - you won't hurt her or damage the training just because of a couple smacks on her rear - obviously don't overdue it or use it as a cure for your frustrations.
Sorry this post is so long, just some suggestions, if you have questions i'd be happy to talk more, or here is a good article about lungeing http://www.liverystable.net/lungeing_article.html
...good luck to you.