Nandini does not seem to be a person who can have feelings just for anyone. First time she must have felt something was for Rajdeep and he must have killed them immediately. I also don't think she had any feelings for Manas who was crazy about her. So technically Kunal would be her first love.
I think the very first time she was impressed with Kunal, was when he treated her and gave her advice. At that time, she did not know he was Mauli husband but she was still attracted to his goodness. Even Rajdeep commented on it. Once she knew, he is Mauli husband, he became untouchable in her mind. But Kunal was the first person who showed her, how a husband should treat ones wife, as the only men she knew were Rajdeep and her father and both were same. He was the one who showed her that women are respected and the with each passing interaction he became her pillar of strength, her comrade and she started depending on him more than Mauli.
I think the tuning point was when he defended her in front of the whole society which no one had ever done before for her. Second was his trust in her to become independent . hence , when she started the cooking class, she only wanted to be successful so his trust remain intact. But at that point,she didn't know her feeling consciously because how can you ever think about your friend's husband in that context.
Her realization was in the form of that baba or may be her subconscious ( we will never know) and that point it still might have been infatuation because she was still able to suppress her feelings and was Ok with Kunal avoiding her. Her downfall was thinking he had died. That was when she realized that her feelings for him are more intense then she gave credit for. She punished herself in same way Rajdeep used to punish her and I think she expected Kunal to mock her as Rajdeep would do. If that would have happened she could have been able to kill her feelings , forget them But Kunal was understanding, sympathetic and concerned only about her pain, feelings and tears. Men in her past have always been selfish but Kunal was different. For the first time there was someone who had given importance to her feelings rather than their own and thus she knew that she will not be able to kill her feelings or suppress them. At that point, she decides to walk away.
This is why I do not think her feelings are superficial or they will ever go away as we never forget our first love. this is just my POV.