My recommendation would be to stop using short or 'cut-down' binders from now on. Find a full-length that you like and which works for you. I would suggest either a 998 (that being the happy-medium between the extreme and the lesser constricting options) as a starting point else an appropriately sized 997 (although I'm personally not a fan of the 997; I hate them. I'm sure I'd hate the 983 even more for the same reasons but anyway lol).
Just as an FYI (and this does have a very relevant point), the sternum is utilized in the Glasgow Coma Scale (GCS) assessment because of how sensitive it is to discomfort and pain from stimuli (e.g. the sternum rub, where the knuckles are raked over it to determine levels of consciousness. It is so uncomfortable or painful that even a coma won't prevent the unconscious person to react involuntarily; unless its a deep coma or brain death). Just as (or perhaps more; for me its more) sensitive, there's the 'Xiphoid process' just below the main body of the sternum and it is precisely right in the junction between the sternum and the diaphragm. This is most likely what is causing you the discomfort and pain with your short or cut-down binder. So it is not going to stop unless you stop using those binder types.
Here's a decent depiction of what I'm talking about:
http://www.crossfitsouthbay.com/wordpress/wp-content/uploads/2013/04/diaphragm.jpgMy older 974 bunches up between the 'pubic floor' and the lower belly but it never causes me undue discomfort and
never pain. If there's pain, its not alright and you need to nip it in the bud. And actually, those 974's before they altered (more accurately, ruined) them worked exceptionally well for me despite D/DD's and while passing (goes to show you don't need 997 or 983 to pass even with large size there). If I'd had a crystal ball, I'd have bought them out before they completely 'changed' them... its a real pity.
Yeah so, the goal with binding should always be a pectoral contour but NEVER to try and achieve 'flatness' or 'as flat as possible' because its not realistic beyond A-B, and the more constriction it takes the more potential for problems (and serious problems) there are. I would never, ever recommend using a short binder anyway given that all of them end right over the diaphragm. Problems, problems, problems. The full-lengths just do not cause as much discomfort because they have a larger distribution area for the pressure and don't put it all right over that diaphragm.
The decision is ultimately yours but that's my recommendation and exactly what I would do if I were in your position.