Give her your attention (especially at the field)
If you take your girl paintballing you gotta put together her set up for her, otherwise she'll be overwhelmed and frustrated. Also, never sprint off down the field leaving her by herself.
Balance your time.
If you plan to play paintball for a day then set aside a day with her, too (or afternoon, whatever). Basically, for every one day of paintball you should be willing to plan a day for her too. Make sure you plan your day with her in advance. It makes her feel special knowing you're thinking about her. Then, when you go to play paintball she won't care so much since she knows she has dibs on you on another day.
This is easily the best advice anybody has given...
I second Rockout...he speaks the truth.
That's what my ex did. He got everything set up for me, gave me the better of his markers and masks, made sure everything was comfortable and fit ok, refused to play against me on walk-on days (though, I would go off and play without him while he tinkered with his equipment at "base"). And then, as I stated earlier, he researched and bought me my own mask. In all, it didn't seem as daunting when there was so much support. It also made me feel like a priority and that spending time with me was important, so sharing his passion gave us something to bond over. (Incidentally, I got him into firearms. So it wasn't a one sided relationship).
If you can't beat 'em, join 'em. Or get 'em to join you. It's about making HER feel like she's important...and if she's even a little bit open to it, make it a big deal that she's joining you. It'll make her feel important to you.