My mastectomy wound has opened almost entirely. Will this heal/fill in on its own as the PS tells me it will? (Photo)

I'm not sure that you had a mastectomy. It appears you had a lift and possible reduction. If that is the case then the area can likely slowly heal in. If it is a mastectomy and there is an implant then that is a concern. This would be best addressed with your surgeon. Best of Luck!

You have to trust your surgeon as your surgeon knows what was done and how your implant (if it was used) is protected.  Attempting to close an open wound is truly just an attempt as it most likely will break down as yours did.  But if there is no implant exposure or implant, your wound will heal with continued wound care but it will take weeks for this to happen.  Trust your surgeon and you should be seen regularly until you are 'out of the woods'. 

It is always difficult to be dealing with such an open wound after surgery. If you indeed had a mastectomy, I would need to know what type of reconstruction you had. If you had an autologous tissue (flap) reconstruction, and there is no implant, then this wound will heal with local wound care. It will likely take weeks. If there is an implant present, I would be much more concerned as the risk of infection would be greater. If on the other hand, your surgery was a breast reduction and not a mastectomy, then you should also heal with time. Your surgeon already attempted to recluse this wound unsuccessfully so  additional surgery would likely fail. It is difficult, but be patient. Once your wound heals, allow more time to soften the scar and you can consider a scar revision at a later date

Just a small point, but from the pictures it looks like you had a mastopexy (breast lift) not a mastectomy (breast removal usually for cancer). Your wound will eventually heal either on its own or perhaps with further surgery like a skin graft. If a reclosure has already failed, I wouldn't recommend trying that again. You may need revisions later after all is healed.