What’s the hardest thing you had to give up, even though it was working?

One of the hardest lessons in business. Just because it's working doesn't mean it's worth it. If what you are achieving is costing you something that you can never get back, then it simply isn't worth it. And this can be doing it at the expense of your personal life, doing it at the expense of your company's core values. The lesson is make sure that when you achieve the goals that you set out to achieve, that it didn't cost you more in the process.