Sri Hanuman did not receive any shrap or curse, due to which he forgot about his strength.
The authentic text on Ramayana is Valmiki's Ramayana. It does not contain any such incident.
We can notice that in the entire Kishkinda Kanda and in Yuddha Kanda, Sri Hanuman acts at the orders of Sri Rama, Sugreeva, Jambavanta, and will not show his strength unnecessarily, give unsolicited advice, acts on his own. He remained in the side lines, working either under Sugreeva or Angada, as the case may be till then.
- After his banishment by Vali, Sugriva was residing on Rishyamuka hill, as Vali did not dare to enter there. Sri Hanuman and other few Vanaras used to follow him.
Due to his long association with Sri Hanuman, Sugriva knew the greatness of Sri Hanuman. As Sri Rama said that Ravana flew in Southern direction with Sita, Sugriva chose to send Sri Hanuman with the troop, headed by Angada, in Southern direction.
गतिः वेगः च तेजः च लाघवम् च महाकपे | पितुः ते सदृशम् वीर मारुतस्य महा
ओजसः || ४-४४-५
"With your unusual mobility, unstoppable celerity, unremitting
vivacity, and untold ability you are identical to your father Maaruti,
the Air-god of marvelling dynamism.
Observing Sugriva praising Sri Hanuman and reposing faith in the latter, Sri Rama gives his ring to Sri Hanuman for showing to Sita.
तत् एवम् प्रस्थितस्य अस्य परिज्ञातस्य कर्मभिः | भर्त्रा परिगृहीतस्य
ध्रुवः कार्य फलोदयः || ४-४४-१०
"By that reason Hanuma must have good track record of the tasks he
accomplished earlier, besides, he is specifically chosen by his king.
As such, this Hanuma will certainly bring task to fruition."
अनेन त्वाम् हरिश्रेष्ठ चिह्नेन जनकाअत्मजा | मत् सकाशात् अनुप्राप्तम्
अनुद्विग्ना अनुपश्यति || ४-४४-१३
"By this emblematic ring, oh, best monkey, Janaka's daughter Seetha
identifies you to have reached her from my proximity, without
apprehension."
- Angada and other vanaras loyal to Vali might not be aware of Sri Hanuman's prowess, as the latter was a close associate of Sugriva and was away from Kishkinda, alongwith Surgriva, for longer duration. However, Jambavata knew the prowess of Sri Hanuman, being senior member of Kishkinda kingdom.
Vanaras got exhausted and felt helpless and Angada and other vanaras even went to the extent of resorting to fast unto death.
बंधनात् च अवसादात् मे श्रेयः प्रायोपवेशनम् | अनुजानंतु माम् सर्वे
गृहम् गच्छंतु वानराः || ४-५५-११
"Fast unto death behoves me than trammels and tramples, thus you all
be acquiescent to my proposal and you may go back to your dwellings."
सुग्रीवम् चैव निन्दन्तः प्रशंसन्तः च वालिनम् || ४-५५-१८ परिवार्य
अंगदम् सर्वे व्यवस्यन् प्रायम् आसितुम् |
While disesteeming Sugreeva but esteeming Vali, all of the vanara-s
have gathered around Angada deciding to sit down for fasting unto
death in step with Angada.
- After Sampati's narration about Sita, the major vanaras explain their respective capacities in jumping over the ocean spreading over 100 Yojanas. Everyone, including Jambavantha were found to be unfit for the task. Though Angada can do that task, he was not allowed to do so being the crown prince.
As everybody got dejected, Jambavanta praises Sri Hanuman and gently advises to achieve the task. As Sri Hanuman respects elders and duty bounded, he started showing his real strength.
That is all.