How to read my girlfriends whatsapp messages without her knowing?

This is a really bad idea. If you feel you have to do this, the trust is already gone. That's the bigger problem here. Trying to spy will just make everything worse if she finds out. You should just talk to her about what is bothering you. Feeling like you need to check her phone is a big red flag in the relationship. You need to deal with the trust stuff first.