Hypothesis 1:
My guess is that he was in love with someone but no one in the family approved. Perhaps he had gone to Sadhna for support and she may have said she will help him. However, she may have gotten scared and backed out once pressured by the elders. This is probably the betrayal that Murli refers to. Sadhna probably felt very guilty for what had happened especially seeing that Murli still has not moved on. She probably didn't want to repeat her mistake and so is singlehandedly supporting LD/Radha. Of course, Murli would be upset that she is supporting LD's love while she failed to support his.
Hypothesis 2:
Sadhna may have supported Murli's love early on but later found out that the girl was evil/gold digger/cheating. So to protect Murli, she may have later said no to the marriage. Murli may have misunderstood the circumstances and didn't see why his mother suddenly changed gears and rejected his love, hence the betrayal.
Either way, I like Shymali though so I hope that Murli/Sadhna have a long conversation and bring closure to the subject of his ex-girlfriend. They should also discuss what is currently happening and why she is supporting Radha/LD. Perhaps he will see things from a different light after such a conversation.
Anyone else have any predictions??