I am lieing to my girlfriend about a tribble sickness

I have a girlfriend and i really really love her. We are planning to get married. The thing is, that i have a very rare sickness called Marfan Syndrom, and it will kill me about when i'm 40 years old. My girl dosent about it. She knows i'm sick but she dont know what will it do. I'm afraid to tell her because i'm afraid that she will leave me. I'm about to tell her soon, will she forgive me?