Mimosas, I really do know how you feel and I feel for you...It is horrible trying to cling on to whatever glimmer of hope we can and I too have people saying I shouldn't bother even of my ex was to suddenly come running back.
Personally I think my situation is slightly different in that it appears her decision is final and I fear she may have even lined someone up to save her from being lonely (maybe just a fear of mine). I will never ever get my head around how someone can seemingly change her mind from wanting everything and making plans to ending things within a day or 2. It has almost felt like the shock of leaving someone in a crash or sudden event.
What I am trying to do is move forward, purely to save my own sanity, by just seeing what life is like without my ex in it. There's no hiding the fact we had a lot of rows, usually over trivial matters or things I felt were double standards in that she was OK to do something but I certainly wasn't. Looking ahead, I will try to chat to new people and pick myself up much like following all the no contact guides you read about online. If I don't hear from my ex in the next few weeks I guess I will have my answer regardless of whether she is suffering from depression (although she has never ever admitted it, despite going to the GP and being given anxiety tablets).
At the end of the day, the most important thing in all this is your own health and happiness so you need to try and make sure you're doing all you can to stay healthy and keep your mind occupied. Keep your chin up x