Well this is what I think based on the facts so far:
Shiv is in love with the portrait girl whom he had met when she was 14.
He seems to not know who she is because he is always speaking about her in the third person even to Ipshita. So I dont think it is Ipshi.
He has a dosh in his kundali by which his life is under threat unless he marries someone with strong kundali like Gauri. This is why he keeps getting the recurring nightmare of someone (may be death) chasing him. But by helping shiv, gauri's kundali is weakened and she will die.
After she dies, he will come to know who is the person in the portrait. So he will be able to marry this girl and be happy. This is his 'khushi' as referred by his mom.
Now comes the trick Nikhila is planning to play. She is planning to reveal Ipshi as the portrait girl after all this. For this she needs some one's (perhaps Ipshi's mother's) help. Shiv as of now does not know that Ipshi will be shown as the portrait girl. He thinks that divine providence will reveal the girl to him after Gauri dies. I think therefore nobody knows who the portrait girl is but Nikhila is making sure that Shiv believes it is Ipshi. Ipshi may also be aware of Nikhila's design and is a willing party because she loves Shiv.
Shiv is feeling guilty because he may have agreed to the marriage only because he wanted to find his love (the portrait girl) and that is not possible till he marries gauri and lets her die. But after the marriage he felt guilty and tried to get rid of her. But unfortunately for him she fell in love with him and is refusing to leave.
Nikhila and Shiv know that if she comes to know that he married her and put her life in threat only for the selfish reason of finding who the protrait girl is then she may leave him. This will affect Nikhila's plan. Shiv may also be unhappy that he cannot find his love but is willing to make that sacrifice but Nikhila is not about to give up her plan to bring Ipshi into his life. That is why she is blackmailing him into not revealing the truth.
Do I make sense?