Hi, so if the Form Mode is New it should reset and be blank. However there is also a Reset function that you can pass the name of a control. It will then reset to the default value of the control.
In terms of editing, you need to ensure that the item of the form is the item whose values you want to be populated in the Form.
Hopefully the following shows some helpful concepts:

So above on the left we have a gallery looking to the same SP List that the Form on the right is hooked up to. The submit button is literally just doing SubmitForm(FormName). Now that in itself and with the Form set to a default mode of New, once submitted it will revert back to a blank empty form to be filled out.

Notice default mode on the right.
I make an entry and click submit, first screen shot. Second shows the state of the form afterwards.

So here this has actually done what you have stated and this is because the form has not been reset and is still in edit mode

So one thing I could do here is have some logic such as in the new button

Pressing new takes me to a state of

Now if we go back and Submit again, except this time we will change the submit button logic to:

Now for the same example, new entry is made and submit is hit


The above is now blank as we ensured we set the Form to new. So your first issue is probably because the Form is still in edit mode. Now in terms of editing the right item, in my example I am using a gallery and when I click to go into the item, it is going to enter the form in edit mode and set a variable that is populated in the item property of the form.



Now there are many ways to achieve the above, one quick way of seeing how you do this is to create a Power App from a SharePoint List. But hopefully this lays down the following key concepts that will help:
FormMode
Setting the Item that exists in the data source of the Form whilst in Edit Mode.
Also look to things like LastSubmit, if you want a quick reference to what was Last Submitted.