Can I regain trust? (27m)

Hi all, I got cheated on by my girlfriend of 4 years (25f) and even though I love her and want to be with her, I cannot get past it. I have invested a lot of time, money, emotions and effort into our relationship and finding out that she was cheating broke my soul. It was during a time when I was thinking of proposing.

When I found out, I got angry for a short time and then I expressed all my feelings to her and we had long discussions and arguments about her actions. Now, she was not physically with someone else, but she had a secret guy on social media that she would chat with, laugh with and even share ‘sexy’ photos and vids of herself with. You may not consider that cheating, but it is in my definition so please don’t try to redefine it.

Our discussions ended with her sincerely apologizing and crying and telling me that “it meant nothing.” I believe her because our relationship is so deep.

However, I’m having to deal with a lot of insecurity and unease. When she goes out, I fear that she’s cheating. When she’s on her phone, i am not sure if she is talking to a guy. When she looks at a guy in the street, I cannot guarantee what she was thinking. All these thoughts are really hard to deal with. You see, I would have bet you a million dollars that she would never cheat on me and I got hit, and hit hard!

I know that I cannot drag this with me if we are to continue our relationship, but I sincerely do not know how to move forward. Please do not suggest that we end it. I want to hear from you if you were ever able to build back trust and to know the way forward.

Thanks!