I need your ab help

UPDATE:  Ok ya'll, I really do need your help.  Went to a water fitness class last night I'm training in for work and it killed me.  They did a lot of dryland stuff like situps, pushups, planks, and seated flutter kicks.  Not to mention this move called a Hellbender.  It's not that those things are super hard in and of themselves, but I couldn't believe how weak my core was and how much it was holding me back.  Yes I did 22 real pushups in a minute, but I shouldn't have pushed myself to do them knees up, my stomach was bulging.  And while I haven't done sit-ups in a long time (don't really believe they're better for you than crunches done well, especially on a ball), I hated how weak I could feel my core was while doing them.
It's remarkable to me that I can go out and run 30 miles in one shot and not have it be a drop dead kill me workout, yet my core still isn't strong.  I imagine how much better everything would be if it was.  Help!

Yes I'm posting a picture. Please don't feel bad or hate. It's really discouraging to me when I talk about myself (on my own blog especially) and it de-motivates people or makes them depressed. NOT my intention, I don't do anything I do to be better than others - where I come in in a race is a different story ;) You know what I'm getting at.
Anyway, yes my abs look pretty good but they have a big weakness. Literally. And it's not my stretch marks. See that diastisis or abdominal separation about 2 inches above and below my belly button? It's a good 2-3 fingertips wide and no amount if flexing (which I am doing by the way, it was for a photo shoot job I didn't get) closes it completely.
I've had one since my 2nd baby but it hasn't been this bad since this last baby. I have to be cautious what core work I do. Anything that makes that area bulge up is bad, pull-ups and prone planks for instance or full supine leg lifts. As I get stronger I'll get more range of motion with the latter and likely be able to do regular planks and pull-ups again but while it bulges I have to pay attention or it could get worse.
The condition is a separation and weakness of the transverse abdominal a, a deep cross running muscle group as their name implies. One thing I know is good are pelvic curls, rolling the pubic bone upward toward the ribs. Holding that (also holding the abs in toward the spine) for 10 seconds makes you aware of the area I'm trying to work. Any ab work I do I occasionally watch my abs or put a hand on my stomach to feel if the gap is closing or not. Closing is good, widening or bulging bad.

Here's where I need your help though. I really need to close that gap down. It's not just an aesthetic thing, it's a strength and function thing. So, what are your suggestions? Have you dealt with this too?