Flying to the US without an ESTA [duplicate]

I have a British passport and live in the UK. I was visiting Canada a few weeks ago and crossed the border (by car) to the US and got a 90 day visa.

I am now back in the UK but the visa is still in my passport (it wasn't removed at the border).

On the ESTA application website it says one is not eligible to apply for an ESTA if they are currently in possession of a visitor's visa.

Will I be okay if fly there with my current visa?

EDIT: I should mention, I have a visitor's visa, but this new trip will be for work purposes

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What happens if you travel to the US without an ESTA?

If you forget to apply for your ESTA, then you will not be allowed to travel. Usually, you will be stopped at your departing airport as you will not have the required travel authorisation to enter the US.

Do you need a physical copy of ESTA?

You don't need to print off or take your ESTA information with you when you travel \u2013 it will automatically be sent to your carrier. It's up to you whether you update your travel information on the ESTA system for future trips.

Can I use the same ESTA twice?

An ESTA can be used for multiple entries into the United States. Should an individual find themselves in a situation where they wish to visit the United States repeatedly beyond the 90 days permitted by ESTA, then a B-1 Business Visa or B-2 Tourist Visa may be a more suitable alternative.

Is my ESTA still valid if I have a new passport?

What happens if my passport details have changed or if it expires? You must submit a new ESTA application and wait for the visa to be approved if you receive a new passport or if any information in your passport has changed. Otherwise your application will be rejected or you will be denied entry to the United States.

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Karolina Grabowska, Malcolm Garret, Matthis, The Lazy Artist Gallery