Like the legendary smackie9 once said:
" If it doesn't feel right, it's not right"
My cousin was in your situation too. Identically the same other than the fact she's a female. There was this older guy, but he was married and had kids.
Him and her were in a long relationship all this time.
They went out just like you and her, but it escalated multiple times and she ended up having sex with him consistently.
She is not a natural cheater but she found it too heartbreaking to tell her boyfriend. They were about to get married just like you.
After one year of consistent cheating, her boyfriend found out via someone else. And I can tell you he was three times as brokenhearted as he he would be if she had told him right away.
You can not just erase this other girl from your mind.
I have ZERO relationship experience, but I believe that honesty always leads to happiness in the end.
Please don't lie to your girlfriend. She shouldn't be a second choice