tl;dr: I see two good options: be direct, explaining to your girlfriend your desire to have the experience of living on your own, or be oblique and talk about living alone in a way that allows for a discussion.
The direct approach:
I love you and want our relationship to keep progressing, but I feel like I need to have experienced living on my own, at least for a while.
The direct approach is attractive because it is clear, leaving less room for (mis)interpretation on your girlfriend's part. Even if you are concerned that she will feel hurt, those concerns are non-unique to this approach. Any sort of lie, evasion, or excuse carries the exact same risks, plus the risk of additional hurt as a result of your indirection. If what you are worried about is that she will be upset you are not moving in together, and she in fact would be upset, there will be no way to avoid that if you live by yourself. I say this not to suggest that you are guaranteed to hurt your girlfriend, but that the effects of your decision will exist no matter what approach you take if the overall scenario is one she doesn't like.
The indirect approach:
I don't recommend this, because there are a thousand ways that your girlfriend could read things into the situation that you did not intend, and you won't necessarily be privy to that information (and therefore won't be able to account for/improve things). But, if you don't want to inform her of why your preference for living alone right now exists, you can casually talk about your new apartment, plans you have for it, etc.
It will be important to be ready for a conversation-- if your girlfriend objects to things, or has questions, you will need to address them or else risk appearing dismissive and uninterested in her. That will cause additional strain on your relationship, beyond this issue in itself. You may want to proactively ask about her feelings, to avoid her quietly nurturing some unhappiness, but this will quickly turn into (1) anyways, in which case you may as well be direct from the outset.
An additional risk to being indirect is ending up with your girlfriend as an informal roommate. If you maintain separate residences, but she comes over to your new place five nights a week, you may not get the "living on your own" experience you're wanting. And if you have a problem with her coming over so often, you'll need to explain why (failing to do so again risks lots of misunderstandings, which won't help your relationship). That, again, brings us back to (1).
Background:
Your worries are understandable, but are probably less pressing than you imagine. Living independently is a valuable experience, and time to yourself is important.
First, the social expectation that you are "supposed" to move in with your girlfriend by now is far from absolute. There are a lot of people that would find it scandalous and inappropriate for you two to move in together while unmarried at any point, and a lot of people that would think it's crazy you hadn't moved in together after dating for a much shorter period of time. The only perspectives that really matter are yours and your girlfriend's.
You're not required to live together at any point while dating, and my personal experience is that while cohabiting is pretty common among people by the time they reach your age many of those people regret doing so as quickly as they did. That you feel you are "supposed" to move in together because of some arbitrary criterion is a terrible reason to move in together. That probably won't remove the sense of pressure you may feel, but to the extent that you are able to not consider that feeling in making your decisions you will likely be better off. There is nothing to feel ashamed about here.
Second, while it's important to tend to your girlfriend and your relationship with her (those are separate considerations, by the way), you also need to tend to yourself. Having a need, or even a strong preference, isn't something you need to dismiss only because it's "just" for you personally. That's not to say that you should pursue your own self-interest at all times no matter what, but you don't need to feel guilty about wanting something for yourself.
The wrinkle here is that you have been "slower to launch" than your girlfriend. While living outside of your parents' house will be a new experience for you, your girlfriend has already done so for years. There are a lot of benefits to staying with your parents as you have done (presumably convenience and financial benefits), but it's unreasonable to expect that you can claim those benefits without any tradeoffs at all. If it turns out to be really important to your girlfriend that you move in together soon, you may have "missed" your opportunity to live alone while still dating her. Living alone is something that you want, generally; relationship items are things that your girlfriend needs from you, specifically. These aren't symmetrical.
If your girlfriend is fine with you living alone for a year or two, after which you start living together, then everything's great. If she's eager for your relationship to advance to living together, she may not be excited to delay things to accommodate the conveniences you've already enjoyed. A major benefit of the direct approach is that you can find out about her feelings immediately, and address them as needed.
Your desire seems reasonable to me, but your girlfriend's desires count as well. If you living alone at this point bothers her, but you insist on doing it anyways, then it's a natural consequence that your relationship will suffer a bit. It boils down to whether or not she wants this now, and if so how badly she wants it. Talking to her is the only way you're going to be able to determine and address these things.
