This has been said numerous times but it’s on you to make sure she knows you have options, and are willing to walk at any time.
Most have said you need to demote her to FB or leave, which would probably be the best move (if you’re having doubts this early on, it will manifest itself in your actions and is usually a downward spiral).
HOWEVER, if you choose to stay in it, here is my advice, which worked wonders for me in my last two relationships…
Make a point of “innocently” mentioning girls who you talked to during that day. Could be a coworker, a barista, anyone. Use their name if possible:
“Marissa at the office complimented me on my shirt today, said the color matches my eyes. I don’t really think so tho.. do you?”
“This is so funny. I was at the grocery store today and this ditzy blonde girl was staring me down with the do-me eyes, and she ran her shopping cart into the display. I was trying so hard not to laugh”
You get the idea… and these work much better if they really happened to you. I’m not advising you to lie. Basically any time a girl shows you interest, make a point of telling your girlfriend about it, but in a “you won’t believe the funny thing that happened to me today” type of way, and make it a good story. And when she is like “that girl is totally into you” (works best with a coworker or someone in social circle) be like “no way, she has a boyfriend, she just talks to me all the time but there’s no way she’s interested in me, we’re just friends”.
This works even better if you can get girls to give you IOIs out in public.
“Why was that girl smiling at me, then immediately gave you a nasty look?” or “That girl over there has made eye contact with me like 7 times now, do I have something on my shirt?”
Stir up a little jealousy, let her know you have options. I didn’t do this daily, maybe once every three days or so. If you’re in a “Netflix and chill” type of relationship, this is especially important. She needs to know other women find you attractive. She will find you more attractive for it, and hopefully won’t stray…
Also, been there. I looked through a non-official-but-basically-exclusive FWBs phone one time, and saw she was talking to another guy (he seemed lame and was shit-ily leading the interaction, but still… hurt). Decided I needed to manage it better and when my untrusting ass looked again months later… nothing. It’s fixable. But is it really worth it? You might want to just focus on bettering your pickup skills so you can get those higher-caliber girls someday - or you risk ending up single and out of practice in the future… like me.
Best of luck brother,
JT