Hello Rekha, 

The curse that kept Radha Krishna separated despite their true love is mention in Brahma-vaivarta Purana.

The main reason behind this curse is the quarrel between Sridama and Radha.

One time, in a rasa-mandala in a secluded place in a great forest in Goloka, Lord Krishna enjoyed pastimes with Sri Radha.

Overcome with bliss, Radha could not understand herself or anything else. Leaving her unsatisfied in the midst of their pastimes, He went to enjoy amorous pastimes with Viraja, another gopi.

Seeing Lord Krishna embraced by Viraja in the jewel pavilion, Radha’s friends at once told their mistress. Hearing their words, Radha became angry and lay down on Her bed. Her eyes like red lotus flowers, the goddess wept bitterly. The great goddess said to them: “Show Him to Me.”

If you speak the truth, then come with Me. I will give Krishna and this gopi the result they deserve. When I punish her who will save her? With her quickly bring Lord Hari’s other lovers also.

She took a chariot to reach that place. Once she reached there. At the entrance she saw a handsome, smiling, lotus-faced guard accompanied by a hundred thousand gopas.

There she saw Sri Krishna ’s dear friend named Sridama, seeing him, goddess Radha, her eyes now like red lotuses, angrily said:

Go away! O servant of a rake, go away! I will see what kind of beautiful lover your master has now!

Hearing Radha’s words, strong and fearless Sridama, holding a stick in his hand, did not allow her to enter.

Their lips trembling, the other gopis angrily pushed the servant Sridama.

Hearing a great commotion from the people of Goloka and understanding that Radha was angry, Lord Krishna disappeared.

Hearing the word “Radha” and seeing Lord Krishna disappear, Viraja became afraid. By practice of mystic yoga she gave up her life. Her body at once became a river circling Goloka.

Sridhama was always for Krishna, and he could not bear to hear Krishna insulted. Sridhama faced Radha and said, “If you scold my Lord even once, I shall curse you.”

After hearing these most powerful words from Sridama, Radharani became even more angry. Her lips were trembling, Her hair became loose, and Her eyes became red like fire. She immediately rose and came out of the house, and approaching Sridama.

She told him: "O foolish one! Super foolish one! Servant of the debauchee Krishna! Listen to Me. You think you know everything!! You think I do not know your Lord! O fallen one in Vraja! Is Krishna only your Lord? Is He not my Lord? I know you always praise the divine father and blasphem the divine mother. As the demons always blasphem devatas you are always blaming Me. That's why now I curse you that you become a demon. You leave Goloka and take birth in demon species. This my curse. Let us see who can protect you from My curse!"

Sridama could not digest this and he too cursed Radharani in turn.

He said :

"Manu?ya iva kopaste tasmatva? manu?i bhubi bhavi?yasi na sa?deho maya sapta tvamambike”

Means : "You are getting angry like a human, hence you'll become a human. You'll become a human without any doubt due to my curse"

"Gokule prapya ta? kru??a? vihari?yasi kanane bhavita te var?asata? vicchedo hari?a saha puna? prapya tamisa? ca golokamajami?yasi”

Means : "In Gokula, getting the company of Krishna, you will enjoy pastimes in the forest. Then, you'll experience a separation of 100 years from Hari. After that, you'll again get His company and return to Go-loka."

After saying this to Sri Radha, Sridama offered his humble obeisances to Her and left to meet Krishna. He explained everything to Him and started crying.

The Lord told Sridama: "I can see that. Now you go to earth planet. You will appear there as Shankhachuda, the king of demons. No one will be able to conquer you. After five hundred yugas you will give up the demon body and by getting stuck with trident of Lord Mahadeva. By My mercy you will return to Goloka dhama."

Sridama pleaded: "Please never deprive me of devotional service unto Your lotus feet."

After Sridama left Sri Radha approached Krishna with tears in Her eyes and narrated the whole thing to Him. Krishna pacified Her.

Thus Srimati Radharani took birth as daughter of Vrishbhanu and married Abhimanyu.

Hope it helps!