I've had a look at your DB alongside the map you sent previously and I think your approach is inappropriate for what you want to do.
I would suggest your primary actions will be focused on the employee, customer and supplier tables, with the person table simply an input to them.
When you add a new record to one of those tables, you will either create a new person or link to an existing one.
As a general rule, every form should be bound to a datasource. There are cases where one would use unbound forms, but not in your situation - so bind the Person form to its table.
Instead of using Text Boxes on your forms, consider using Combo Boxes to select from pre-defined lists. So the Employee PersonId would be in a combo box with its data source as a query to the Person table. You can construct the Drop-down list to show Lastname, Firstname. If you type a name which is not in the list, the NotInList event will trigger (providing the combo box properties are set correctly). From the NotInList event, you would open the Person form and populate the record as required. When the form is closed, you use the newly-added record as the data source for the combo box. You can find examples of how to do this in Access VBA help.
I think the way your forms are designed does not help your business process. It appears as if they are simply a view of the table, with no enhancement - by this I mean the PersonId (for example) is shown as a number, where the actual name would be more useful. Again, the combo box will solve that problem.
I suggest you start by describing your business process in words (avoiding technical jargon entirely). Describe what needs to happen when a new customer is to be added to your database (e.g. 'Customer detail to be captured, primary (secondary/tertiary) contacts names to be assigned ...' etc. By doing this, you will start to map out the logical flow and structure of your DB.
When considering your table design, think about the attributes in common between customers, suppliers and employees and list them as a group. For each type, list the unique attributes as separate groups (I'm talking about pencil and paper here

). This might give you a different approach to your table structure, where you might combine suppliers and customers, for instance (but you might not).
I have amended your customer form to change the PersonId to a combo box to illustrate that basic method. Try doing the same for your other tables to understand the mechanism. Pay careful attention to the properties of the combo box in the Data and Format tabs. I haven't included the NotInList function here, but as I said earlier, you should find help on this elsewhere (it's just about knowing what you want).
I don't know how helpful this is and it might seem a bit daunting. If you can break your problem down into steps, then this community will help with each step.