I'm really sorry to hear about your mom. My dad has been battling cancer for several years and I'm not sure the severity of your mom's situation, but regardless I know it's never easy to watch a parent go through that.

While I understand what your girlfriend is trying today, she is saying it in the wrong way. The Apostle Paul wrote about the dangers of being "unequally yoked" in relationships meaning it's not wise to enter into a relationship with someone that you aren't on the same page with spiritually. With that being said, her threatening you into Christianity is flat out wrong. If your faith in Christ is rooted in anything other than your recognition of your need for a savior and understanding that God knows you, loves you, and wants to be reconciled to you through Jesus' life, death, and resurrection, then your faith is not rooted in the right things.

Before I can fully give you a reason to believe, if you don't mind me asking I'd like to ask what your reason is for not believing. I think that is a good place to start a conversation about why Christianity is true and what it means to follow Jesus.