She is clearly cheating. It's very obvious to those of us with experience with infidelity. She's gaslighting you hard too, so you are going to have trouble trusting your gut.

She felt bored or claustrophobic or unhopeful in the relationship with you because of the monotony of the pandemic and her unrealistic expectations for the relationship. So she went elsewhere. She may have set it up with the new man before she left, but not necessarily. She is definitely spending nights with him. 26-year-old women do not regularly sleep over their friend's house three nights a week. She was spending every weekend with him and now the overnights are becoming more frequent, as often happens in these situations. She is avoiding your calls when she is with him.

Textbook infidelity.

Been there my dude and if it's your first time it can feel awful.

I recommend you start the process of accepting that she is not the woman you thought she was. She is immature, selfish, dishonest, and lacks integrity. What you ought to do, is try to understand how you missed all the red flags while you were "madly in love." When you revisit the relationship with fresh eyes, you will see the flags everywhere.