I cheated on her and feel terrible!

I made a mistake and cheated on my long-term girlfriend after we became a long-distance couple. We were separated for too long and I know what I did was wrong, but it was a struggle and I couldn’t cope. She doesn’t know about this because it will hurt her and this relationship will end immediately, which is why I don’t know what to do. We love each other a lot and have been discussing marriage but the guilt of what I have done is making it hard for me to even face her on video calls. What should I do? I want to be honest, but I can’t because I know this will not end well for us. I am heartbroken about what I did but I also know she will never forgive me.
You did something unforgivable and are looking for a way of moving on without addressing it. No relationship that is built on lies can survive beyond a point, because you owe your girlfriend as well as yourself that level of honesty. Yes, she may leave you, but her reasons will be justified. To brush this aside and assume you will get over it is always a possibility, but you have to ask yourself if the toll on your mental health is worth the lie. Also, if you genuinely love someone, withholding the truth from them is hardly the way to show it. Think about whether this is fair to your girlfriend and do what is right for her, instead of focusing on your guilt. If you have committed a crime, you have to learn to accept the consequences too.

A few days ago, someone I have been dating said she can’t commit to a relationship because I was not enough for her. This sounds transactional and I don’t know what she means. Should I keep trying to convince her?
She has been clear about not wanting a relationship and even if her reasons make sense to her alone, she is entitled to her decision.