Wait until you're around 25 and better off financially and in terms of your maturity. It's fine to talk about how special she is to you- but marriage is a lot bigger than you realize, and you will stay together if you are meant to be lifelong partners. When you are ready, you will feel a lot more solid than you clearly do now. You may have made the right choice- but you're not ready to take the leap yet. Talk to her about how you feel.
You are a bit young to propose still- and I would strongly suggest that this be a discussion between you and your GF first. Women were once considered property- first of their father's, and then of their husband's. Thankfully, that no longer exists, and your only request should be of your GF. It would be disrespectful to imply that she is not a whole person- and adult- who can make that decision for herself.
If she is old enough to marry, she is old enough to make that decision on her own. Don't make her feel as if she doesn't make her own decisions by asking her parents' permission for anything, other than to use their house for a party.
She isn't property- she is a woman. And if you haven't had discussions about the hard questions in life- whether or not she wants children, in which religion they might be raised, where you would live, whether or not you would take in a sick relative, whether or not you would move for a dream job of hers or yours, whether or not you would terminate a pregnancy of a terribly deformed child, what you would do if one of you were paralysed...
If you haven't had those discussions, and this is more of a formality than a surprise, you aren't ready. So talk about all those things, and be mature about this- or you aren't ready. She can walk across the street without their help. She can make her own life decisions.