My boyfriend (28m) has been dishonest with me about his co-worker, and I'm unsure whether I (25f) should forgive him.

To begin with, we've been in a relationship for four years, and my boyfriend has never exhibited any signs of dishonesty until now. I recently spent a month abroad visiting family, leaving him to spend the holiday season on his own, which made him feel quite isolated. Most of his current friends are people I've introduced him to, and he doesn't have a large circle of his own, so he didn't have many options for socializing during my absence. While he spent Christmas with a mutual friend, he had no plans for New Year's Eve. While I was away, he socialized with a group of coworkers. He told me they gathered at our place, and he really enjoyed it, feeling optimistic about forming friendships with them. However, one night he texted me saying he had been invited out for drinks, and it wasn't until recently (after I went through his phone, which I know was wrong) that I discovered he had actually been hanging out alone with a female coworker, rather than a group. Initially, it was meant to be a group outing, but when everyone canceled, she was the only one available. Additionally, he had invited this coworker to spend New Year’s Eve together, suggesting they do a countdown and cook at our place. He never mentioned this to me beforehand. Although he eventually canceled the plan, stating he felt it was inappropriate, he still went out for drinks with her alone afterward. I had my doubts and made them clear since my return, especially as he had been texting several female coworkers. When I asked him to establish boundaries, he was initially defensive but agreed. However, the next day, I found out he texted that same coworker again to go out for drinks, and although she suggested bringing another female coworker along, he expressed that he enjoyed their time together just the two of them. We eventually talked about everything, and he pleaded for my forgiveness when I mentioned considering a breakup. He explained how lonely he felt and that he had hidden the outing from me to avoid upsetting me. He claimed his intention was just to enjoy friendship and gossip about other coworkers, insisting he had no romantic feelings for her and found her unattractive. He believed he wasn’t doing anything wrong, aiming to prevent conflict about spending time with her. Being a kind person, I could see how he might come across as trustworthy to other girls. He has since promised to be more cautious and set boundaries: no more solo outings, and he agreed to an open phone policy. However, later that same night, this coworker texted him to say she was feeling down and wanted to grab a drink. I asked him to prove his commitment to change, and he declined the invitation and canceled their next outing in a clear manner, which she didn’t seem to mind. Now, I’m left wondering if I should give him the benefit of the doubt or walk away, considering he lied to me, and I can never be certain that he won’t do it again. It bothers me that he prioritized his loneliness over our relationship, seeking conversation with anyone, without considering my feelings or the implications of his deception. After four years together, this is the first significant betrayal I’ve encountered from him.