Persuader,
When I was 19, I was dating an agnostic-leaning-atheist. At the time, my faith was weak and I was questioning. We started living together, and I got on the Pill. The sex was great, we were madly in love, but something started going wrong. I felt like the life was draining out of me. I started to feel like the only time I was happy was when we were having sex - but afterwards, I felt horrible. I didn’t really want to not be a virgin anymore. I started wanting to live according to my faith again, but was struggling with the wretched reality that I loved my boyfriend more than God, and that’s how I was living my life. I was crying daily, and my boyfriend was incredibly torn and guilt-ridden.
Then he did the things that changed the entire direction of the relationship: He quit having sex with me and encouraged me to go to Confession. We continued to live together because we had shared credit cards, bank accounts, lease, etc., but we were chaste. I could receive the Eucharist again! He started attending church with me - first on holidays, then every Sunday. We got engaged a few months later, and have now been (mostly) happily married for 4.5 years.
So while my situation wasn’t identical (I stayed Catholic, although I had a LOT of “If you exist” conversations with God during that time), it was very similar to what your girlfriend is experiencing. My advice: Stop having sex. Encourage her to stop using the contraceptive (especially since many have side effects of depression) and to talk to her priest. See if she will confess her lack of faith (and sexual relationship) in the sacrament of Reconciliation - because Reconciliation is now precisely what you want for her. Consider that the “life” she had wasn’t just a psychological state, but was something real and concrete - a true gift of the Holy Spirit. You CANNOT help her regain her faith unless you encourage her to life like a Christian, including chastity, so giving up sex is absolutely necessary - or you will be saying one thing with your mouth, and another thing with your body.
And if you really want to help her get her faith back? Pray. Pray for her, desperately. If God exists, He exists regardless of whether or not you believe in Him. So you might as well pray. If He doesn’t exist, the worst case is that you will be talking to yourself or to empty air. If you are in a public space at the time, people might look at you strangely - I speak from experience

You don’t have to lie and tell God you believe in Him. Just lay it out straight - you are desperate, you don’t know what to do, you don’t believe in Him, but if He does exist, you want Him to help reopen your girlfriend’s heart. And consider praying for yourself a little while you are at it

I assume that if God
does exist, you’d want to know the truth and believe in Him. So tell Him that. He can change hearts. Give Him whatever little willingness you have to work with Him-if-He-exists, and trust that He can do the rest.
And consider praying with your girlfriend. Even without faith, you can still pray. Just talk to God-if-He-exists together, and seek Him. He will answer you.
I hope this helps. I really feel for you. I’d ask my husband to stop by and offer his $0.02, but he’s pretty shy and not one to give advice. I’ll ask him for his thoughts as well, though. Since he’s been in something similar to your shoes, he might have some insight.