Please help!!! This is what my girlfriend said to me

I promised to get my girlfriend an iPhone 11 pro max because her phone was bad. Few weeks later I got myself an iPhone 11. She saw the phone and was like oh cool, then I told her I would buy her own any moment but I keep delaying it and teasing her just because I’m still thinking if this my girlfriend who had cheated on me truly worth this phone?!

This girl I’m talking about had cheated on me, I saw her WhatsApp chat telling a guy I love you, and all that. I confronted her and she said she was sorry, that a friend of her gave her number to the guy without telling her and she kept talking and met the guy once at the mall. 
I told her I called the guy and he told me he knew the girl had a guy because of her body movement when she visits him. But my girl swear with her mother in the grave that he had never visited this guy at his house, that they only met once at the mall and after then he keeps sending her money while at school. 

The truth is that I forgive her because she swear with her mother in the grave. That one really touched me that is why I believe her that she never visited the guy in his house which means they never had sex. 

But I said to myself, this relationship will never lead to marriage for her to hangout with a guy while still dating me. I honestly lost the trust I had for this girl. Now I’m planning to breakup with her in a calm and coded way because I don’t longer trust her. But because she currently has no phone, I said I will buy her the iPhone and then use distance issue to breakup gradually with her as I’m planning to go on a month trip soon. 

But surprisingly, my girl was with me yesterday and I was teasing her that, since she has been using my iPhone 11 and even went home with it for a week, there was no need buying her the iPhone 11 pro max. And she replied:

No, it’s the iPhone 11 pro max I want, but you keep delaying this, do you feel pained to buy the phone for me??

This her response didn’t go down well with me, I keep wondering why she said if I feel pained to buy her the phone? I told her I’m not happy with such question, that it means if I buy her the phone she won’t appreciate it because she would feel like I bought out of pain and she said no that she was sorry, I shouldn’t take it that way. 

She is 20yrs old. 

Please advise me, I love her and care for her that is why I just decided I will get her the phone and break up with her, so it doesn’t hurt her too much. But due to this I decided never to get her the phone again by giving her excuses until I travel.