The title explains the situation, but there are a couple of details that you need to know about to have a better understanding of the situation:
She (23, hair-dresser and part-time model) and me (24, college student) have been in an on-and-off relationship for 4 years. Although we have often split up and found each other again, we never lost a strong (physical) desire for each other.
Her understanding of a relationship is to go on vacation together. I myself hate "vacation". I would rather travel, but that's not possible with her because she prefers hotels and restaurants over sleeping in the woods and a meal from the supermarket.
She has hinted every day for 8 weeks that she wants to go on vacation with me, repeating the date when she wants to go at least once a day, but always in a rather friendly manner.
She is clearly overworked and states often that staying overseas with me would cure this.
My passion is entrepreneurship and I've been saving my thin student-worker wages to bridge the time of unemployment when starting a startup. This being said, going on holidays would set me back financially quite a bit.
Although I like her very much because she has a very innocent heart, I do not think of her as my future wife. From experience (advice from successful entrepreneurs) I know that a startup rarely tolerates any relationship on the side.
Telling her this, however gentle and respectful, will trigger a very sad / disappointed reaction in her.
The question is therefore: How do I tell her all of this while being as adult and respectful about the situation as possible (what choice of words, what situation, how to behave if she starts a massive guilt-drama)?