I think I am understanding your requirements. Its looks like you want to store all the past values of Last_Note__c field into the Notes 7& Attachment section correct!?

Here is a trigger that would do the job:
 

Trigger NewNotesCreation On Opportunity(After Insert, After Update){
    
    List<Note> notesFinalListToInsertForAfterInsertOpps = New List<Note>();
    List<Note> notesFinalListToInsertForAfterUpdateOpps = New List<Note>();
    
    If(Trigger.IsInsert)
    {
        For(Opportunity Opp : Trigger.New)
        {
            If(Opp.Last_Note__c != Null)
            {
                Note nt = New Note();
                nt.Title = Opp.Last_Note__c;
                nt.parentId = Opp.Id;
                notesFinalListToInsertForAfterInsertOpps.add(nt);
            }
        }
        try{
            If(!notesFinalListToInsertForAfterInsertOpps.IsEmpty()){
                insert notesFinalListToInsertForAfterInsertOpps;
            }
        }
        catch(Exception e){
            System.debug('Thrown Exception while inserting notesFinalListToInsertForAfterInsertOpps is:: ' + e.getMessage());
        }
    }
    
    If(Trigger.IsUpdate)
    {
        for(Opportunity Opp : Trigger.New)
        {
            
            If(Opp.Last_Note__c != null)
            {
                String oldvalueOfLastNote = Trigger.OldMap.get(Opp.Id).Last_Note__c;
                string newValueOfLastNote = Opp.Last_Note__c;
                
                Boolean lastNoteHasChanged = (oldvalueOfLastNote != newValueOfLastNote);
                
                If(lastNoteHasChanged){
                    Note nt = New Note();
                    nt.Title = newValueOfLastNote;
                    nt.parentId = Opp.Id;
                    notesFinalListToInsertForAfterUpdateOpps.add(nt);
                }
                
            }
            try{
                If(!notesFinalListToInsertForAfterUpdateOpps.IsEmpty()){
                    insert notesFinalListToInsertForAfterUpdateOpps;
                }
            }
            Catch(Exception e){
                System.debug('Thrown Exception while inserting notesFinalListToInsertForAfterUpdateOpps Is:: ' + e.getMessage());
            }
        }
    }
}

This is what the Trigger Does:

Let's say you create an opportunity with name of "Burlington Opp - 1" and provide value in field "Last_Value__c" as "Burlington Note - 1" and hit the save button, at this moment you should have new automatic note record created under "Notes & Attachment" section with note title as "Burlington Note - 1" 
Now, if you change "Last_value__c" on "Burlington Opp - 1" to "Burlington Note - 2" then you should have another automatic note record got created with title as "Burlington Note - 2" etc.. 

Sample Image For the behavior:



Hope this helps!