Broken

Apologies for the rant in advance, just feeling pretty bad, any feedback would be welcome. I have suffered with bad depression for a few years and have just recently been dumped. When we first started seeing eachother we discussed how important it is to not intentionally hurt others and to always be honest and open. However it's now come to light that I was probably being used as a rebound and the pain is absolutely excruciating. I found out the day they dumped me that they were asking for their ex back. I got in touch asking for an explanation, they said I love you but can't be in a relationship and promised they didn't want the ex back and they finished with me as I made them feel like an option and not a priority. Which I know isn't true as the ex told me different, I stupidly messaged them on facebook. Why not just be honest? Would feel so much better if they just told the truth or even a sorry would suffice but I'm just ignored and cut off with nothing. If I had experience and knew the warning signs of someone in a rebound relationship I would never have bothered with them! Why won't they just admit what they were doing and apologise?