I'm about 18 months into my own start-up and only marriage has been better at revealing my blind spots and opportunities for growth and self-improvement. The first thing I would say to a young entrepreneur is: "Congratulations. You're about to learn more about yourself than you ever imagined possible. Embrace the opportunity to grow as a person."
The other answers here are very good. The weak habits I noticed in myself:
1. The weakness of not being able to admit your weaknesses. I am mediocre at organization and management (meaning I can get by, but I don't excel and I'm way too slow). It took 18 months to admit this to myself and others. My bosses at other jobs pointed out this weakness over the years and I always disputed it out of pride. Admitting this weakness allowed me to change the business model and operating plan to focus on what I do well. For example, I hired a bookkeeper. I also made the decision to get out of direct project management (outsourcing instead). Admitting my weakness does not diminish me; it makes way for the business to grow.
2. The weakness of ignoring cash. Cash can be a scary thing when you're starting out (because you don't have a lot of it). Scary = negative feelings. Negative feelings can get an entrepreneur down in a hurry. So, I spent most of my first year ignoring cash and only looking at my budget once in a while. I had clients and a positive balance, so why worry about cash? Abruptly losing my biggest client to a sudden change in management jolted me awake. Instead of looking at cash as a scary thing, I'm learning to think of a puzzle that only I have the guts and wits to solve (with help from my bookkeeper). Ignoring cash may preserve my delusion of being smart; tackling it head-on and overcoming the challenges is what makes me truly smart.
3. The weakness of selling to the wrong people. I wish I had back all those hours I spent (and dollars, too) on people who would affirm and applaud my business idea but had neither money nor reason to buy from me. It could delude myself that I was having sales meetings, but no sales is all the proof you need that you're wasting your time. I'd go so far as to say that "networking" is a dangerous word for an entrepreneur because it can be just as much of a cop-out. Get in front of people who can and will buy your product or service. It's do or die.
I hope some of my mistakes are helpful to you and your clients. If you want to know more, I'm happy to listen!