Generally I would say stay tough know you're not alone and that you will either find someone else or move on together.
But I have no idea about either of your relationships dynamic, what stage you're in of your life or what experience you have.
So stay strong, play video games and maybe have some beer.

If she gets on like that then you should reconsider your relationship with her

In my experience there'sdifferent kinds of flirting. Theres the kind that's just harmless everyday back and forth, often with people you may not even be attracted to.. And then there's the more serious kind. If it's the second option, and she's done this more than once, and she knows quite well it makes you uncomfortable.. My opinion, personally, would be to re-evaluate whether the relationship is worth it. It's hard to comment on it properly since, as derpderp said, I don't know enough details. But that's just my face-value opinion.
As for inspiration and motvation-- I find those do tend to fluctuate, especially when things are going poorly in other situations. It may be a good idea to step back for a bit, or even focus on just one project. You don't even have to work on them necessarily, but it might be a good idea to keep a notepad around just to jot down any random thoughts you may have related to your projects-- no matter how innane they seem.

As derpderp also said, taking some time just for YOU might be a good thing. Everyone needs breaks. Stay away from your girlfriend and any other stresses if you can for even a little while. Do some thing you love or haven't done in awhile. Even something simple. Go to your favourite store, play your favourite game. Go for a walk in a park (<- nature has a very uplifting effect on people, even when they don't typically enjoy it. The color green alone has been proven to help reduce stress levels, and that's just a start). Hang out with a friend or go to a pet store/dog park if that's your thing.
All these things in your life matter, but that doesn't mean they matter more than just making sure you yourself are okay first. Your projects were meant to be something to enjoy. They'll wait for you to come back to them. Or start new ones. It's okay to lose motivation. You're not a machine. You're human.

Kill your family

Use the machete

Eat the bodies and gain their strength.

Use the strength to build an altar

Well that escalated quickly...

I'd have to agree with @xvarnah on the relationship this about evaluating from here. I think that severity can vary because each couple has a different relationship, but If it's clearly been an exclusive relationship and she's crossing boundaries, I'd make sure you discuss it with her. (Communication saves lives sometimes.) I think that if y'all have both knowingly been in an exclusive dynamic and she's talking to 8+ guys it sounds like she's not valuing your relationship. Coming from a girl, the only time a gal in a relationship has that many side talks is if she's browsing. Take time for you, but really evaluate her level of commitment and see if it matches yours. Be as clear about it as you can. If she's playing around and you aren't, I think you should consider if it's worth it. If It doesn't bring you happiness in the long term, it's probably not worth it.