You have 2 problems the lie and the age difference tackle the lie first, tell her your real age now then give her a few weeks to change her mind about you if she wants to. It is entirely possible that she has already worked that out, young women are not stupid
Men around 40 are very attractive to many young women they are more settled, mature and have more money, from the security point of view they are a good risk.
One of my nephews on my husbands side aged 41 married a 21 yr old two yrs ago. He is a regular guy sociable, hard worker, moderate drinker, nothing bad that I know of. She is not a typical 21 yr old, mature well beyond her years, works at least a hard as him also sociable and a moderate drinker and vows never to have children, despite the age difference the seem a good match.
2 years on they always socialise together and it is clear that she is the boss and he seems happy to accept that, you can never really tell but I think they will stay together.
So from your point of view it all depends what your girlfriends goal is and will that fit in with your ambitions. So assuming she still wants you when she knows your real age, make sure that her life plan is compatable with yours.
D