I have yet to speak to a single damn person who will admit that they are currently in their stupid immature faze. And yet there is no doubt that people have stupid and immature phases. How can it be that everyone has these phases, but no one seems to believe that they are currently in one themselves? If a person realized that they were acting immature, do you think they would continue to act that way? Probably not.
Perspective means taking a step back, so let me say this. At the age of 19 spending $2000 on a girl you've dated for three months is not normal and not healthy. Granted, part of that is because most 19 year olds don't have $2000 worth of disposable income (except the ones that are stupid enough to rack up an enormous amount of credit card debt, and while I hate to break it to you, if that sounds like you, then you are not out of your stupid and immature faze. In fact, you've ratcheted it up to a whole other level, because that shit may very well follow you for the rest of your life. For most people $2000 is a lot of money... A LOT OF GOD DAMNED MONEY. Seriously, do you know how much better most people could make their lives with just $500 to throw around? (assuming they used it right) Not to speak of four times that much.)
Second, she may not act like or think of herself as a "material girl," but most people would feel uncomfortable accepting that much from a relationship so young. Strike that, most mature people would feel uncomfortable accepting that much from a relationship that young. Hell, just today I checked some lady out, she had cancer and obviously was not well off, and when she went to pay she couldn't afford all of her groceries. The people in line behind her, who were obviously pretty damn well off, stepped up and paid for her, even though she begged them not to. If a cancer victim whose funds have probably been eaten up by medical bills and who is obviously down on her luck can beg someone to not give them $40, then the fact that your girlfriend (who I assume is well off and capable of taking care of herself) would accept $2000 worth of gifts speaks fucking volumes.
People are incredibly good at performing the mental gymnastics necessary to validate the choices they made to benefit themselves. You "want" to spend $2000 on this girl because you think it'll help you find love. And I'd bet my whole fucking bank account (which you'd better believe isn't even close to $2000, but is still enough for me to get by) that she has convinced herself that she accepted those gifts because she loved you. But I'll tell you this, very few people really understand themselves (myself included), they don't really know how they feel, they don't really know who they are, and they don't really know what the hell they're doing. And that's especially god damned true when you're 19.
I'll give you the same advice (though I'd also bet my whole bank account that you wouldn't listen to it), which is to have some perspective. Just take a step back and think about all the relationships that fail. You're hardly the first person to spend exorbitant amounts on girls that they should not have spent near that much on. And you sure as fuck won't be the last. No one thinks that they're going to be that person. And yet we all know people who are. How do you know that you're not that person? Someone has to be. The fact of the matter is that if I'm right you probably won't listen to me to start with, my words will fall on deaf ears, and you'll do what you want anyways.
That, I'm afraid, is life. A whole mess of shit you really can't do too much about, despite your best intentions. So you know what. I hope that I'm wrong here (it wouldn't be the first time). But I wouldn't bet my bank on that.