I haven't been in the mono/poly type relationship you're talking about but I can comment on secondary relationships.
I'm secondary to my secondary (Possibility) who also happens to have two primaries.
We both get things out of our relationship that we can't get from our main relationships. It sounds like that's what's happening with you, you're giving her the things she can't get from her primary because he just doesn't work that way.
What you do need to consider though is whether you will be happy always being secondary. For now that might be what you need because of your limited time due to education demands, etc. but looking into the future do you see yourself as being happy as a secondary.
There's always the option of you finding a primary for yourself too but you've said you don't see that happening in the near future.
Just some thoughts.