Is what you're feeling irrational? Yes (more on that in a sec).

Is what you're feeling normal? Also yes.

You're young and this is your first serious relationship, and you're not alone in feeling this way. It's perfectly normal to think to yourself "hey, this love and sex and relationship thing is awesome, I wonder if it can be just as awesome/more awesome with other people!"

But normal doesn't mean rational. It's normal for a parent to want to smack their kid for screaming, it's just not ok to actually do it. While there's nothing wrong with wanting to leave a bad or unfulfilling relationship, it's important to understand your own motivations for doing so.

Think of it like this: what's your best-case-scenario in looking for a new partner? Someone who loves you? Someone who you can trust and rely on? Someone who encourages and supports you in your personal and professional growth? Someone who makes you laugh? Someone who you have great sex with?

Does your GF fulfill those important qualities for you? (not necessarily the ones I listed, but the ones that are important to you?) Does she make you happy?

If so, you're being irrational. You're worrying about replacing a great partner with someone who might be just as great if you're lucky. But the fact that there are other people out there shouldn't make a difference.