Definition: Given a real-life anecdote of a complex ethical situation, generate a suitable title that describes the main event/root cause of the situation. Imagine that the given text is a social media post, and you have to write the title of the post, so the users can decide to read the main text or not.
Input: I don't usually post or anything, so I'm sorry if this is wonky. Just looking for insight from some people other than my mom.
So my gf and I are both juniors in highschool. We've been together around 2.5 years now. She's extremely insecure and kinda untrusting of me. I've never known why, she's never been willing to talk about it. I've never cheated nor do I lie (I'm actually brutally honest a lot, I have trouble controlling it, I fully know sometimes it doesn't go over well and pay the consequences for being rude). We're each other's first everything; relationship, kiss, sexual hoo-ha.
I'd have hoped after that long together there'd be trust but it never fully developed. Deep down I fear she's projecting because of some things that happened freshman year, I can go deep into that but it doesn't feel relevant rn. So whenever we're hanging out outside school, she always asks to go through my phone. I always oblige, nothing to hide here. I don't text other people really, don't have porn or anything on here. It's never been a problem. But things keep getting crazier. Like if it's a weekend and I take too long to answer a text, she demands I tell her what I'm doing and breaks down. We go to a small school and I can't even look at another girl, but like we all grew up together and know each other well, we all are friends, what's the big deal. I'm not into them. She's just walking by and I noticed.
Anyway here's where I made the potential asshole move. So I run indoor track, and kids who do sports hang out in the cafeteria before practices start. A week ago, after we got out of class, we went to the cafeteria and I dropped my bags off and went to the bathroom. She waited at the table I set my stuff down. I keep my phone and wallet in my backpack, just sort of randomly tossed in there. She wouldn't know where they were, she'd have to fully search my bag to find them. When I got back to the cafeteria she was holding my phone and glaring my way. She says, "What the fuck is this?" and hands me my phone. I felt afraid like I did something stupid and somehow forgot and hid it, but I looked and it was a conversation with my track coach from earlier that week. He's practically family to me, and she knows that. It didn't matter that it was a conversation with him, it was that I never told her we texted. I thought that was ridiculous, he's literally my coach, a 50 y/o man with a family, I'm 17. I was also upset that she did it without asking, and deliberately when I wouldn't be in her presence. She told me what she did was acceptable and normal and I was wrong. I do not feel that way, I feel she overreacted to something harmless and invaded my privacy.
Nothing else was said, and after I got home I told my mom. I needed advice. I never opened up to her about our relationship before this. She agreed with me, and told me to change my password so she couldn't do it again, and I instantly did so. I felt better, I got a lot off my chest. Well we hung out this weekend, I figured it'd be fine because a few days passed and I had a meet Saturday so we didn't talk much that day. At a random point she grabbed my phone and I just watched as she typed my old password in, it failed, tried again, it failed again. I told her I talked to my mom and she said she was out of line and told me to change my password. Which I did. Well now I'm a POS who is being sneaky and has things to hide. Right then and there she took out her phone and changed her password. I don't really even check her phone so I didn't really care. But that was a problem too because now I don't care about our relationship and am ok with her "talking to other guys" or cheating. She's pissed and I don't know what to do, she's never willing to talk through things. It's impossible for me to bring things up without it blowing out of proportion.
So I'm wondering, AITA here? Was I wrong in changing the password? Did I overreact to her going into my phone?

Tl;dr Girlfriend went through my phone, found a convo with my track coach. Got upset at me for not telling her. I changed my password so she couldn't do it again, and now apparently I've got things I don't want her finding out.

I feel like there's a chance the story seems like something is missing, like someone will feel it's one-sided. I didn't try for that but I didn't want it crazy long. If there's a hole or something and someone has a question, just ask. We had a snow day but I have an appt soon so I'll be unable to check this for a while

changing my phone's passcode just so my gf wouldn't be able to get on it