I'm jealous of my girlfriend's work colleague because they have so much in common

I (21M) have a girlfriend (20F) that gets along extremely well with a work colleague Mark (20M) and I'm jealous of their bond.

We have been together for about a year now and she's everything you could ask for in a girl. Smart, kind, funny, beautiful, friendly. I can feel that she really loves me and I loved her back yet I can't stop having these thoughts.

For context, when she was in 11th grade she signed up for this NGO that offers online courses such as digital marketing, web design and project management. She would learn stuff online for 4 months and get a diploma at the end that shows her skills. After trying all courses she enjoyed the whole thing so much that the community team behind it accepted her as some kind of ambassador. Her role was to help new members integrate and answer questions they might have about the programe. When we got together she persuaded me to join but I wasn't interested. At the same time as her, Mark (20M) became an ambassador too. They've been friends for 2 years now and co-leading weekly meetings every Saturday at this hub that belongs to the NGO in which they host game nights, small parties or help the members with their weekly homeworks. Because this is such a big part of her life and she enjoys doing this she and Mark talk a lot about what they should do the following week for their weekly sessions, joins calls with other community members or have inside jokes about everything. When it's the two of us she tries to explain what's happening but I don't get it and I never bothered learning what she talks about which might have been a mistake. I'm ashamed to admit but after seeing how they act with each other face 2 face by laughing with the members and Mark especially I confronted my girlfriend and even went through her phone but all her conversations are pretty much professional. Still, even I can admit they would make a good couple because they have many things in common such as loving little kids (I can't stand them), beings religious (I am too but I never go to church or anything like that), enjoy cooking (I'm a disaster in the kitchen) and more.

I wish I could stop this feeling especially because she told me she only sees Mark as a good friend and he had a ton of girlfriends since they've known each other and nothing happened.