From my perspective-
It sounds like she's got a fear of "what if" regarding you dating. Of course-there's no way to know what if-until if happens.
But-my husband had similar issues and at times I've struggled with similar as well.
My first suggestion is-FIND A POLY GROUP SHE will talk in. There's several facebook groups and there may be one in or near your area. If she meets other poly people she can be friends with-that can be supportive of helping her work through her fears as they crop up around this-that will help A LOT.
Also-consider attending one of the poly camps that happen all over the world. Even if you have to plan it for 2-3 years out, start putting money away and get on their email lists so you get updated info as it comes out.
Spend more time TOGETHER-going to social events that "mix it up" THAT ARE NOT PART OF THE TWO GROUPS SHE YOU CAN'T DATE FROM. For example, a dance class that teaches some sort of dance where people switch partners. Activities where she can get used to seeing you socializing with other women AND she is socializing as well. It will help her see that your interest in her isn't diminished by attention from other "potentials".
Finally, and most people struggle with this advice-but it is still my personal advice; get rid of the closet-and do it with her around.
Talk about open relationships with other people-AND her. Not just when she isn't around.
There's a mental set that says "Everyone will think...." but the truth is that most people don't take time to think much of anything when they find out someone else is in an open relationship. Primarily because they don't really give a damn about SOMEONE ELSE'S relationship.
The more she see's that people DON'T CARE-the less she will feel the need to "hide" AND the less fear she will have that if you DO meet someone else-suddenly the world is going to look down on her because of it.
"Love As Thou Wilt"