I didn't want to say anything before and color this, but let me say this. I became pretty devout in my faith (Catholicism as well), after years of doubt and wavering and disbelief, etc. It was rough, cause I sorta understand your girlfriends perpsective on that. it's not a dealbreaker to date other people of other faiths, but at the time I was exploring and deepening my faith, I lost alotta friends and a pretty solid (albeit not as solid as I thought) relationship, because of it.
Openness and honesty is the best way to handle it. Sounds like you guys are strong in that department. Respect is also important too. There are ways to like...be able to talk about "faith' or other things related that might be an easier way for you to bridge that gap, without a conversion. There are a number of books written not just by "Catholics". Maybe reading those books would help? PM me and I'll suggest some books to read if you want, not just like "self-help" stuff but actual literature.
I can admit, coming from a Catholic family, that sometimes that pressure is applied, but it doesn't seem like that's a dealbreaker or actively driving a wedge. It's just rough because (from my perspective), something as personal as faith, is a deep part of your life and it informs how you act and believe in things, and nowadays it's a pretty big choice to make a commitment to it.
I applaud you for your commitment as well to her. I know this isn't an easy process, and there are alotta people that would call it quits based on that. Not that that's a bad thing, but it's just a different choice. Idk, like I said I'm pretty devout myself so if you do have any questions to ask that are either weird to ask your GF or you just want another perspective on feel free to PM me. Best of Luck.