At 20 years old, you're too young to be tying yourself down to any one person. Having what I assume is exclusivity with your girlfriend since age 17 is a recipe for disaster.
So consider talking to her about taking a break from the relationship, if this is such a glorious opportunity for you.
Be prepared for this ending your current relationship. It probably will.
As for your objectified coworker, it's stupid to get involved with someone at work. Full stop. You'll poison your working environment. Your coworker is being inappropriate.
But I get the feeling it's not about this specific coworker; it's more about being tied down at an early age. Early intimacy, being super young and spouting on about love and forever are ways that people set up this exact scenario. Or you'll be looking to boink a coworker or the checkout girl at the grocery store or your neighbor's wife or whomever when you're 40 and have been married for 12 years and have 2 kids.
Yeah, that's not so wonderful, either.
But I would still consider talking to your girlfriend about exclusivity, and not with the coworker in your mind at all. Talk to your girlfriend about how things have been, and what her expectations are. She may be thinking she's all set for happily ever after. Or she might be feeling as stuck as you are right now, but doesn't want to hurt you.