I've been with this girl for over a month (know her for over a year) and everything has been going very well, she regularly tells me that she truly loves me and will always be there for me etc.
But today my mind suddenly became quite clear today and I felt something wasn't right. I started to realise that I'm the one who's been doing everything, she rarely texts me first, and yesterday, she's replied to most other peoples messages but mine. I'm starting to have doubts and lose hope because she's never really "been there", and I feel the need to put a bullet in the relationship before it gets worse. Everything she says barely have any backing evidence.

My mind tells me that she loves me, but my instincts tell the opposite. I don't know which to believe. I just feel like she's losing interest and I really want to know the truth, so would it be right to suddenly ask her "if [she] really loves me?" and would it hurt her if she really does (but just doesn't know how to express it).

I guess you had better ask her (why would it hurt her) and get to the bottom of this before this one sided relationship goes any further. It is quite normal to get this out in the open, it is called communication.