I would think about this situation very,very seriously byteme. Only move in with your girlfriend if you are 100% committed to the relationship, as you will be taking on such alot for a young man and you may feel resentful as time goes onand feel that you are suffocating in the house, not just privacy wise but emotionally and financially. I take it that you are a similar age to your girlfriend/?You should be out having fun and enjoying your money. This is such a negative situation to land yourself in with free will when people are screaming to be released from situation where they are bogged down by financial worries.
Your girlfriend's Mum, should be telling her daughter to go ahead with her life, study and do well for herself, instead of pinning all her hopes on being bailed out by her. I do think this situation is unfair to your girlfriend and to you, if you take on the burden.