Hello -Luca, I'm gathering that your girlfriend does not have a problem with your Christianity, yes (or is she aware of little more about your faith than what you just told us, IOW, that you are a Christian who prays and reads the Bible)?
Also, it would be helpful to know if she is part of a different faith than Christianity, if she has no faith at all (e.g. humanist, atheist, agnostic, etc), or if she is actually opposed to having a religious faith of any kind(?)
In general, I would look for/pray for additional ways to bring up God/Christianity/your faith (and what God and your faith have meant to you and why/in a practical sense) in the day by day/moment by moment of your relationship with her. IOW, begin to find ways to make God and your faith a normal part of your conversations with her whenever possible.
Also, if you have not begun to do so already, pray for her salvation regularly.
BTW, I believe that learning to ask leading questions (rather than simply giving unbelievers the facts, so to speak) is the best way to witness, because asking such questions will allow the person who you are sharing your faith with to come to their own conclusions (and they feel less "backed into a corner" as a result).
If you're not sure that you know how to do that, this book will help you:
Tactics, 10th Anniversary Edition: A Plan for Discussing Your Chrisian Convictions by Greg Koukl.
Finally, if you have not been a Christian in BOTH word and deed in your 5-month relationship with your girlfriend, then you need to start to be, for her sake principally, but for yours as well

Apologizing for having not been totally upfront with her about how important/integral your faith is to you/your life, for instance, could be a good way to break the ice and get a conversation started

Praying for you!
God bless you!!
--David
p.s. - as
@Soyeong has mentioned, you need to consider the fact that she may never come to saving faith in the Lord Jesus Christ, and what you will do if she does not (again, for her sake as well as your own).
Romans 10
17 Faith comes from hearing, and hearing by the word of Christ.