Trust issues eating me alive

TL;DR:
18M, been dating my girlfriend (18F) for 6 months. She was close friends with my mate Steve (18M) before we met. Steve now dates her best friend. I trust my girlfriend, but I’ve got trust issues and can’t shake the feeling something’s off—especially after she hung out one-on-one with Steve for 4 hours before Sarah arrived. Nothing likely happened, but it eats away at me mentally. Just needed to vent and maybe hear some advice.

Hey Reddit, I don’t really know where else to put this, but I just need to get it off my chest and maybe hear some outside perspectives, am I overreacting and overthinking this?.

I’m an 18M and I’ve been with my girlfriend (let’s call her Jane, 18F) for about six months now. We were introduced through a good mate of mine, Steve (also 18M), who was already quite close with Jane before I ever met her. Now, Steve is dating Jane’s best friend (we’ll call her Sarah, 18F), so the four of us have kind of become a little social circle.

Steve and Jane were “close” before I came into the picture—lots of calling and texting—but to my knowledge, they never actually saw each other one-on-one. That’s always felt a bit weird to me. Especially because I know Steve really well, and he’s the type of guy who seems to cycle through who’s closest to him every six months or so. That makes me question how deep that connection with Jane ever really was.

Here’s where my own issues come into play. I’ll be honest—I’ve got major trust issues. Not because Jane has done anything wrong, but just because of the way my brain works. My head tells me nothing’s wrong. But my heart? It constantly second-guesses. I get anxious, overthink everything, and end up feeling really down—even when I know I shouldn’t.

Recently, I was away for the weekend and Jane told me she was going to catch up with Steve and Sarah. I was totally fine with that—until I found out she got to Steve’s house four hours before Sarah did. That really threw me off. Remember, Jane and Steve have never hung out one-on-one before, so why now? I’m 99% sure nothing happened, but that 1% of doubt has been eating away at me. I spent most of the day in a depressive state, imagining all these scenarios I know probably aren’t real.

Another thing is—I suspect Steve might have had feelings for Jane at some point. He has a long track record of falling for his girl best friends. He’s literally told me that about nearly all of them. It’s almost comical, but in this situation, it makes me uneasy.

I’m trying so hard not to come across as controlling or jealous, especially as my girlfriend thinks it’s quite important to be openly friendly with the opposite gender. That’s the last thing I want. But this feeling won’t leave me alone. I care about Jane so much, and I want to trust her. But the doubts keep creeping in and it’s affecting how I act and feel day to day.

I don’t even know exactly what I’m looking for here. Maybe advice. Maybe someone to tell me I’m overthinking, or someone who’s been in my shoes. Or maybe I just needed to get this out of my head and into the world. Thanks for