The idea that if you were to date someone else you might find someone better is a siren song that's going to lead you to ruin, friend. It's natural to wonder whether there may be "someone better" out there somewhere, however that's where maturity should step in and inform you that it's foolish to throw away a solid relationship on the hope of something better.
First of all, it's incredibly insulting to your girlfriend, because you're telling her that she's not enough. Don't expect to be able to have such a conversation, and not break her heart. It may literally spell the end of your relationship. In fact, let me be clear:
You shouldn't talk to her unless you know exactly what it is about your relationship that you'd like to address, or if you're breaking up with her.
Second, you need to realize that life is not a Disney movie. What "better" are you hoping to find? Someone who listens more attentively? Who likes more of your hobbies? Who's kinkier in bed?
Unless you know exactly what's unsatisfying in your relationship what are you going to go looking for? And if you do know, then why aren't you taking steps to improve your relationship such that those things are true of it?
The older you get the more you realize that a successful relationship is not about looks (although they do matter), but about how the two of you interact on a daily basis. Because the boring, day-to-day activities comprise 90% of your entire life.
If you have respect, honesty, and the ability to engage in conversations openly, and without judgement, then you already have more than many (most?) people ever find in a partner.
Sure, you'll see some more attractive woman walking down the street, and wonder what it would be like if you could pursue her, but, realistically, the chances of finding someone with whom you click as well as your current partner are very low.
Among other things, it takes years to tease out all of the different aspects of someone's personality, and come to realize whether they're the kind of person you could spend the next decade of your life with, never-mind marry and have children.
And so, I advise you to take a long, hard look at your relationship before you throw it away.
There are legitimate reasons for walking away from a relationship - even a long term one - such as abuse, or the fact that your partner is toxic, or a dead-beat.
If your partner is the mill stone around your neck, always holding you back from trying new experiences, or living life the way you'd like to - and could * - live it, then perhaps it's best to part ways.
* The distinction is important because being told that you can't live like James Bond is just being realistic, not holding you back. Someone telling you not to learn how to snowboard even though it's a dream of yours, however, is probably holding you back due to their own fears/insecurities/agenda.
It's best to find someone who shares your passions, but it's typically impossible to find a partner who enjoys 100% of the same activities as you. Even then, some activities (such as excessive gaming) are a net negative to your development, and your partner may push you to pick up something a little more useful. That's not necessarily a sign that you should break up. It may, in fact, constitute a sign that you need to analyze your lifestyle and aim to do a little more with yourself.
Someone who tries to cut you away from your friends, family, and hobbies, and demands constant, fawning attention is a bad choice of partner.
Generally speaking, if the above are not the case with you, then you probably have a pretty decent relationship, and throwing it away on hopefully finding someone better is silly. However, you need to find that peace within yourself, otherwise anything someone on the internet tells you is useless. Remember that you only live once.