What kind of major project is this where you're days behind and nobody has noticed or asked about it? That means either there's a lot of trust in you, or it's perhaps not as critical to other people as it seems to you. If it's the latter, you might have a pleasant surprise and find there is more time on the table.

Regardless of all that, the only thing you can do is to be honest and let your boss know what's up.

For future projects, even if you aren't asked for updates I'd still suggest giving them honestly. Even if you're behind - no in fact, especially if you are behind.

Admitting you're behind might feel bad, but it's also sharing responsibility. If your boss knows the project is behind then they also have to help do something about it, be that reprioritising or finding help or extending - and if they can't do that then they're also failing at being a competent boss.

Unethical life pro tip: go out on fmla leave immediately.

Ethical life pro tip: learn from this, once you behind you need to speak up.

Adhere to first rule of holes: when in one, stop digging. Go out on leave because you aren't well or speak up immediately and own the error.

You might want to think about talking to your manager and explaining the situation to them. Maybe they can provide options for reaching a better outcome. Also I think they will be happier to be aware of the situation earlier rather than when it's too late.

Call in your manager and explain de situation. Better to be lay off on your own terms.

What Tribbles friend said. Talk to your boss. Starting the project over with someone else won't be faster than giving you time/support to finish it.

Are you medicated, seeing a psychiatrist?

Ive got similar deadline problems. Dont pretend like trying harder yourself will solve this. Also dont think humbly admitting fault is useful either.

For myself i invited more oversight or handshakes with my manager before risk is too high. Something like, "are you on schedule?" Isnt enough for me, ive gotta have that 15 minute screen share where i show how on schedule i am materially. Otherwise, ill lie to myself that all is well, when its not. Trying to demonstrate independent performance isnt worth fucking it up again.

Your manager will appreciate knowing how to get better results. Saying you need help with this sort of task management is admitting weakness, but if its accomodated by safe guards then its all good. Ive fooled myself too many times trying to fix similar behavior, and these are the only successful ways ive found.

Put those check-ins on the calendar from the get-go at useful periods the moment you got something assigned.

Whatever you do, don’t be talked into resigning. Don’t sign any sort of severance without talking to a lawyer, either.

Better to as forgiveness than permission? As others have said, lay it all out for your manager what has happened. If a company doesn't account for missteps in projects and they take longer, it's a pretty poor company.