NTA

I was in a similar position to you a couple of years ago when my mum told me that my siblings and I would be receiving a long-overdue inheritance from my great grandmother who died like 15 years beforehand because the lawyer who was looking after her estate dipped with the money but we were able to get that money back thanks to some legal sorcery. That ended up being around 330k for each of us in both shares and liquid assets.

Mum was adamant we not tell anyone (partners included) until we knew our assets were safeguarded and we felt ready to let them know. My girlfriend accidentally found out two weeks after I got the money because she peeped my balance while I was transferring money for something and she. went. ballistic; she was practically saying the same stuff about keeping secrets and how I could betray her trust - ugly night all around. However we’d been seriously dating for a number of years at this point so it’s a bit more muddy. She went to sleep on it and the next day we both realised that we allowed our emotions to get the better of us and she was ultimately happy that I was able to receive such a lottery ticket in life. She did however say that her feelings toward it might not have ended up the same if I was better able to honour my mum’s wish for it to be a secret.

Your gf, however, is really on some shit; it’s your money and you spent/invested how you saw fit whilst in the early stages of being in a relationship.