One of the lessons I got from Paul Graham is "Do NOT trust your instinct. Startups are so weird that if you trust your instincts, you'll make a lot of mistakes."
Also, pay much more attention in understanding your users well than in being a startup expert.
Stay a generalist, and be deeply involved in all parts of the company. The key is to be like your founders, but even more focused on execution.
Also, don't hesitate to question everything (yourself, the company, the founders, the users) and do not ever leave any detail unanswered.
For me is not getting too attached to my own ideas. I'm working on the first version of the product and a lot of ideas get scraped. I don't care what ideas of mine go to the trash bin, I trust my mentors and the only thing I care about is a successful launch. If advice helps me get closer to it, I do whatever is needed.
You must define your goal clearly. Your goal should be not your product or services. Your goal should be -making money. Remember one thing you promise to your investors is - money.
So what ever comes across your goal you have to separate ways with it.
If your marketing plan doesnt work - abandon it.
If your sales strategy doesnt work - kiss it goodbye.
If your salesmen dont deliver - cut them.
If you tried all strategies but product fails - kick it to the curb and pivot.
If your engineers cant or dont want to pivot -cut them.
If you have pivoted many times but you still fail - save your investors money. Let them cut you. Be ruthless like Chyngis Khan. Let them have another general for their money. Dont waste their money and time. People trust you part of their lives through money you taken.
Adapt QUICKLY. You are finding your feet. You have few tried and tested processes. You have endless amount of decisions to make. You will make less than optimal decisions. So pay attention and adapt quickly.