I just discovered that my partner [22M] messaged someone without my knowledge five years ago. TL;DR

How would you approach this situation, considering it’s been five years since it happened? My boyfriend and I have been living together for three years and have been sharing expenses since 2022. Recently, I discovered that while we were together for just over a year, he messaged a random girl on Discord back in 2020, asking her for revealing pictures of her body. There was no conversation or compliments—just repeated requests for her to send specific images. He sent her pictures of himself, including a sexual image, in an attempt to persuade her to send something back. I checked the timeline to see if we were having any issues at that time, but we were in a good place; in fact, he was texting me while he was messaging her. I had invited him over that day, and we spent time together shortly after his conversations with her. I’ve never suspected him of cheating or flirty behavior, so I was completely blindsided by this revelation. If it had occurred in 2019, when we were going through a rough patch, it might not have shocked me as much. But finding out it happened in July 2020, when we were seemingly solid, made me feel sick. I know it wasn’t emotional cheating, but it still felt like a betrayal given that he was actively seeking explicit exchanges with someone else. The girl messaged him multiple times and he only replied five days later with a casual “What’s up?” indicating he didn’t feel guilty soon after. I confronted him today, and he expressed genuine remorse about his actions, recognizing their seriousness. However, I wanted an explanation, and he admitted he couldn’t provide one because he didn’t remember the details, suggesting it was “insignificant” to him at that time. It felt dismissive to hear him imply he was just trying to relieve himself and that it meant nothing. While we both agree to allow each other to watch porn, the act of going out of his way to exchange intimate pictures with a random person is a different matter. He tried to rationalize his behavior, attributing it to being toxic and young, but he wasn’t excusing it. He urged me not to view him differently because of this, but it’s tough for me to separate that moment from our relationship now, especially since it happened right before he came to see me. He mentioned wanting to keep the lines of communication open to find a resolution, but after hearing that he couldn't explain his actions, I felt it pointless to continue the conversation. While his visible remorse eased my feelings somewhat, I’m still at a loss for what to do next.