Youre requesting illegal info here and besides this tells me that you don't trust her at all so if that's the case youll be better off just breaking off the relationship and moving on. If you do succeed in finding a way and she finds out, youre done with anyhow/
NOT EVEN WORTH TRYING IT. JUST MOVE ON WITH LIFE MAN. HOW SAD ARE YOU...
If you want to hack your girlfriend's messages, there are a couple of ways you can approach this:
Either you can do something really complicated like get some software installed on her phone that will secretly forward her messages. This is 100% possible, but is usually quite expensive and complicated.
The simplest (and most effective) way to get into your girlfriend's messages is simply to look at her phone when she unlocks her phone.
It's the number 1 way that phones and computers are hacked, even though complicated and risky software options get talked about way more (because companies want to sell you these products).
The final point I would make is that, if you are really that desperate to spy on your girlfriend, that would suggest the trust in your relationship is gone.
If that is the case, then you might want to think about why you guys are still together. Sometimes it's better to call it a day rather than discovering something you don't want to hear.