What feels like the most difficult part of starting a company?

Nothing of the options but the need of a solid business plan.

Being able to pivot quickly as you learn. When you start a company with a new idea, you will learn a lot when talking to customers.  You'll learn why they buy and why they don't.  If you're willing to quickly incorporate what you're learning into your solution, you'll have a much better chance of success.

There are many distractions that can slow you down but you can't let it distract you from delivering the right solution to your customers. 

If you've been in a bigger company before, you're likely now doing many activities where you used to pick up the phone to have someone else do. I remember googling an IT issue when I started my business. I missed being able to pick up the phone and have an IT person at my desk immediately repair an issue. These distractions can slow you down, and I'd suggest building a team of contract outside help so you don't get bogged down. It's important to spend time on the first point above.

Solving a problem that people want solved and are willing to pay for

A good team makes it significantly easier to make smart & fast PMF pivots to ensure your MVO comes to life as a great idea. A good team makes it much ea raise funds as well.

I founded 10 companies and in my experience it’s not the idea or money but are people always the hard part.

We can have a great idea and and a lot of money but to run a profitable business you need people with competence and passion that work with you day by day.

You can “buy” people time but not their passion.