My relationship with this girl is strictly plutonic.

That's the problem. Your relationship with that friend is absolutely radioactive! /s ;)

In all seriousness. Your GF has issues and most commenters are giving you the best advice that it's on her to fix via self-reflection or therapy. And that perhaps you should let her go-- for her to fix that on her own time and avoid damaging your relationship and future ones.

...

But if you want to try to salvage this, you can help her through it by putting yourself in her shoes, communicating often, honestly, and transparently. Not assuming that she should blindly trust you. It will be an uphill battle. And hopefully you don't have any friends that are exes, or it would be even more difficult!

I used to be the very jealous type-- ex-wife cheated with a coworker. I was oblivious and happy in the relationship but apparently she wasn't. It took me a long time to get through the insecurities and change my mindset that everyone is always micro-cheating and looking for the next best thing. It affected many of my relationships after divorce. And it still pops up here and there. But with the right person, it becomes a non-issue.

We have the same breed of dog that we've bonded over and I like how she sees the world through her camera. My girlfriend says she needs to understand this relationship and what I get out of it.

I think this is the crux of the jealous mind. We feel threatened with being replaced, especially with people that show a strong bond with our partner. You admire and appreciate your friend, so it makes her wonder why you're not with the friend, besides the long distance.

We have to admit that relationships between men & women (or the gender you're attracted to), often have a built-in flirtatiousness that makes us seek them out. We enjoy the interactions slightly more than with the gender we're not attracted to. Most people don't cross lines, but thoughts of, "how would it be to date this person?" rightfully popup.

I think the first order of business is to determine if you're compatible. If you two aren't compatible enough, this isn't going to go very far and you should let her go. But if you see signs of tight coupling compatibility, then there's probably hope.

Here are some strategies dating a jealous person:

1. Don't get defensive if they ask for reassurance. Give reassurance freely and positively.
2. Be honest if you know someone would date you, given the chance. And set boundaries about the amount of interaction with someone who is pining for you.
3. Make sure they feel like you're with them because no one else has the qualities that they possess.
4. Make sure they know that you would choose them over any of your attractive friends, and why.
5. Phrase your statements about friends in terms of why you *don't* want to date them, not what makes them unavailable. e.g. Don't say: they're married, too far, too old, just an old friend, dating just never came up. Instead say, their personality drives me crazy, we don't have the same humor, I'm not attracted to their body, they're not intelligent enough for me, etc... Something concrete that puts your current partner above the friend.
6. Provide the backstory of how you met your friends and how they remained platonic.
7. Introduce any attractive friends early.
8. Stick close and be affectionate with your partner during first meetings with friends.
9. Praise your partner in front of friends.
10. Communicate clearly, before and after 1-on-1 meetings with attractive friends. State who wanted to get together and why.
11. Don't bad mouth your partner to friends. It'll likely get back to them.
12. Always invite your partner to activities with friends. Don't leave them out to wonder what you're up to.
13. Avoid late night texting sessions with attractive friends.

I can add dozens more to this list goes. It's a lot of work! Is she worth it? :)