Transition to a new job

Yesterday, I interviewed at Arby's and a coupla hours later, I received an automated text saying I'm hired. Now I completed all my onboarding paperwork and am awaiting being contacted by the hiring manager.
The past Sunday, I also gave my 2 weeks notice at my current job. Beginning of May is my last day. I been at this job for 3 years 3 months.

My supervisor said I'm not performing well and this position could be too fast paced and high volume for me. I do sometimes find it hard to keep up. I get distracted, disorganized, and forgetful. I lose focus and couldn't keep tract of what I'm supposed to do. The day before, I left sharp objects in the sink in soapy water and someone cut herself.

Next thing I know, my supervisor told me I need to give my 2 weeks notice...unless I'm confident I could improve. If not, I could possibly be fired. I gave my 2 weeks notice bc I lack confidence in my performance. She said I can come back at any time when things get better. It would just depend on how the interview goes the next time around (whenever that may be.)

How do you deal with this transition? YOu kno, from a professional retail atmosphere to fast food? Oh well, at least my supervisor said we could continue on as friends on facebook and IRL. But still saddening Help me.