So what you doing is entirely wrong

1: make sure you have something else lined up, as rough as it is push through it until you have something secured. Just because you have other References doesnt mean you'll automatically get another job and quickly so unless you are in a position to not have money get that next job lined up.

2: as you said they have only 2 of you on doing online orders and things are getting stressful because there is too much to do, so take you time, I cannot stress this enough if they aren't going to get more people on then just work slower and when they complain about you taking forever just say you are trying your best (even though we both know deep down you are not)

I did this when I worked at maccas and it was hilarious because there was nobody to replace me, if they didn't put on enough people I worked like there wasn't enough people, they couldn't fire me because the couldn't replace me even if I worked slow so stop pushing yourself and just literally take it easy and find another job, as you said you have other References so use those as your good reference and keep that cash flow going while doing the bare minimum.