Seeing all the panels helps people to help you. Lots of info there that might tell others something.
General Q&A note Help us all to help you.
Please give full screenshots with any questions. With the relevant panels open.
Also, include the lecture time and name/number that is relevant to the problem/issue.
The screenshot can be done by Blender itself, via the ‘Window’ menu bar top left hand side.
On that menu drop down is ‘save screenshot’.
Close ups additionally where they help.
That’s the reason why it is no longer mirroring - you don’t apply mirror until you are finished with anything new you might want to take effect across the mirror.
What you could do is turn on xray - select vertex in edit mode and delete one entire side half (not the middle verts though). Then select that “half” object and choose mirror modifier again - but don’t apply it.