Female boss hitting on me, GF wants to breakup 7 yr relationship

For a bit of context, I (28M) have been on a 7 year relationship with my GF (27F) and we have been living together for almost a year now.

However, for the past 5 months, I have been approached by a senior manager (who is 2 levels above me in my company). First she (31F) invited for some 5 minutes coffee machine breaks, then she started a project and choose me for that project, and then for a drink and for dinner.

I thought that I was being valued for my performance and developing a good relation with my boss, however she told me she had feelings for me, and I explained to her that I didn’t feel the same, I had a GF and was building a life with her. After this conversation, she stopped the moves for a while.

Some weeks later, she called me to advise there was an open position for team leader in another department and that I should apply to it. She also told me she could give some tips, and we agreed to meet after work, and I cleared up that it wouldn’t be any date or something, just as work friends.

Long story short, I always told my GF what was happening, however seeing as she reacted, I hid some things from her, and this is where i messed up.

I didn’t tell my GF of all the times I was with her, and lied to her about not being woth her. Also, there were some company events where she was and I didn’tell my GF to avoid a fight. However I NEVER considered cheating, and wasn’t attracted or anything.

My GF saw some messages in my company laptop and found out about it. She moved out and thinks I cheated on her. I have no idea what to do. Any help?