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That's always a tough one, the first relationship, and with the first person you've ever had sex with to boot.
Your first relationship rarely lasts "forever". Very few people go on to marry their first love. The reasons are manifold, and every first relationship fails for its own reasons.
But I guess the underlying common theme is that people just aren't satisfied with the prospect of having one romantic partner all your life anymore. In our times, serial monogamy is the preferred model, and many people feel like they lose out if they don't at least have a few different partners before they decide to settle down with somebody.
And that's both a good thing and a bad thing. The good thing is, having had a few different relationships, you will be more aware of the kind of person you are, and of what is important to you in a romantic relationship. You gain life experience. On the other hand, my nan for example is from a small town in rural Cornwall. And in those days, you usually met your future husband some time around age 18 to 20, and then you got married soon after and stayed together for the rest of your lives. My nan was married to my granddad for 53 years before he passed on due to heart failure, and they spent nearly every single day together during that time. The approach to a relationship was much different from today, in that your concept of it was that you met one person, got married to them, and tried to make things work out with that person for better or worse. Then again, it'd be naive to think that married people didn't end up having affairs with somebody else at some point during such a marriage.
My advice to OP would be: try to figure out what it really is that puts you off about the prospect of spending the rest of your life with your current girlfriend, or at least a substantial amount of time of it. Is it just the thought that you might be missing out by just committing to that one person, or can you actually not see yourself with her at age 40, having spent around a dozen years married to each other with a mortgage and kids? Is it HER that you can't see yourself with, despite your declared love for her?
I kind of have a gut feeling that you will end up separating. Your urge to sow wild oats elsewhere looks strong enough to me that you will eventually give in to it and not look back. But whatever the future holds for you, embrace it.