AITA for not telling my boyfriend I’m moving out after I found out he cheated?

Sorry, this may be long.

I (42F) have been with my boyfriend (33M) for four years, and we’ve lived together for two years. Our lease is up in a week and a half, and I’ve made the decision to move out after I found out that he cheated on me with his best friend.

For context, his best friend, “Tina” (34F), and him were roommates before him and I got together and before we all decided to move in together. He and I were just friends for 8 years prior to committing. During our friendship phase any time she would call and needed him he would drop whatever and make sure she was alright. I will admit Tina is not the brightest and at 34 doesn't really know how to be an adult. She fixes everything with two to three different guys a week and it's made easy because of her profession. Not shaming her just stating facts. Over the years, I had some doubts about their relationship. but he always reassured me that they were just friends, and I trusted him.

Two months ago, I found out the truth. His behavior had been changing—he was distant, secretive, and I could just feel something was off. Yes, I went through his phone and saw the exchanges. I confronted him, and after some hesitation, he admitted that he slept with Tina. He said it was a one-time mistake, he regretted it, and begged for my forgiveness, claiming it didn’t mean anything. The texts say different especially since there were a few texts from the previous day. I went through his phone at like 3AM. He really wants me to stay and work things out.

But I can’t. I feel betrayed, stops, and humiliated. I’m also angry because he didn’t just cheat—he cheated with someone who lived in our home, shared meals with me, had heart to hearts with me, and acted like a friend. They would reassure me about my insecurities about the closeness of their friendship (it's always the ones they tell you not to worry about 🤦🏾‍♀️).

I’ve already decided I’m done. I’ve signed a lease for a new apartment, arranged movers, and planned to leave next week. I haven't told him yet and I don't plan to until I'm gone. Since he goes to work before me, my plan is to pack and move all my things while he’s at work. I'll leave a letter explaining my decision.

This is where I may be the asshole. I know he can’t afford this place or a new place without me. He’s financially struggling, and without me, he’s going to have a hard time finding somewhere else to live. But I'm not responsible for fixing his situation, especially after the betrayal.

Some friends think I’m justified in not telling him, saying that he made his choices and now he has to face the consequences. My sister, however, thinks I’m being cruel by not at least giving him a heads-up so he can make arrangements. I don't feel like he deserves my sympathy. He can have my middle finger though 🤷🏾‍♀️

Oh, yeah.Tina doesn't live here anymore either.

So, Reddit, AITA for moving out without telling my cheating boyfriend, knowing he can’t afford this place or a new place on his own?