By not hiring a senior developer and only paying you, they are spending less money. It should be expected that you will deliver the features slower. That is why you are not getting paid the same thing. Now, it's their job to figure if it's better for them to have only you or to hire someone else.

If you are working your hardest, don't lose sleep over the fact that you cannot meet deadlines.

It is alright to challenge schedules. They may be used to have a certain pace, however that pace need to change if they are not willing to replace the senior dev. that bailed.

That being said, be the senior. Fake it.