How to explain to a manager that the workload is too much?

I work as a designer in a consulting company. I was told to take on another project.

The company doesn’t want me to straight up say to my current project that I had to take another one. I have to basically say that I overestimated my capabilities and won’t deliver what I promised.

I have really hard time with not being straight with people, and also with not being organised, having to break deadlines etc.

I don’t know how to explain to my manager that I feel uncomfortable essentially lying to my colleagues while also taking the blame on myself for not delivering promised work.

There is also the expactation that you should be at 2 calls at the same time, or be on a call while doing designs. I can’t do meaningfull work while being present on a call.

Feedback given from my colleagues to the manager is pretty much always ignored. There are almost no jobs in my field right now. But if the stress is too much on me, I will rather go back to retail. Idk how to say this to the manager without sounding arogant. I know the company needs me, but they don’t take my (or other people’s) concerns seriously.

Advice will be appreciated.

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