How do I express to my girlfriend who wants me to become Catholic that I don'thave interest withouthurting her

I first met my now girlfriend over 17 years ago at work. My girlfriend comes from a strong Catholic background and has stayed the course throughout the years. We were best friends the first 12 years and her marriage fell apart. Long story short we became an item and it was great on my part, but she had issues with me not being religious, but I am spiritual. Guys I treat everyone the way I'd like to be treated and I'm not just patting myself on my back, I feel good about who I am and I express care and love to those around me. We broke up for 3 years ago for other reasons (nothing bad) just our lives pulled us elsewhere, but we recently came back together and this issue came up again. In the past I went to church with her and her family weekly, and I did the same again this past week, but it doesn't seem good enough for her. I told her if I were to convert to be catholic I'd be doing this just for her, not because I was moved to do it for myself and she says she knows and understands this, but she still gets upset. I love this woman and regardless to what happens I'd do anything for her and her kids, but I can't lie and fake something so serious as religion just because she wants me to be that. I don't want to hurt her, but I feel this will continually be something brought up in the future. I don't know if she's ashamed to bring me to church with her not being the same faith as she is or what. I'm well loved and thought of by my friends and family but this comes about as close as I've ever felt as not being good enough for someone.