If a person forgets something in his recitation or makes a mistake in it whilst praying, if it is in al-Faatihah then he has to correct his recitation, because there is no prayer for the one who does not recite it (al-Faatihah). If he forgets something or makes a mistake in it that changes the meaning, his prayer is not valid until he corrects it. 

If the mistake is not in al-Faatihah, then his prayer is valid, because recitation after al-Faatihah is Sunnah and is not obligatory. 

The scholars of the Standing Committee for Issuing Fatwas stated: 

If a person forgets the soorah after al-Faatihah, he does not have to do anything further, whether he is leading the prayer, praying behind an imam or praying on his own, and whether the prayer is obligatory or naafil. This is according to the more correct of the two scholarly opinions. End quote. 

Fataawa al-Lajnah al-Daa’imah, 7/146 

If a person makes a mistake when reciting the soorah or he forgets anything of it, it is not prescribed for him to say istighfaar (ask for forgiveness); rather he should try to correct the mistake and remember what he forgot, but if he cannot do that then he may move on to the verse that comes after it, or he may leave this soorah and start reciting another, or he may bow. If he does any of these things, there is nothing wrong with that. 

The scholars of the Standing Committee for Issuing Fatwas stated: 

If the worshipper becomes confused when reciting a verse and he cannot remember it, there is no reason why he cannot recite the following verse, but it is prescribed for him not to recite in prayer anything but that which he has memorised well, so that he will not get confused a great deal. End quote. 

Fataawa al-Lajnah al-Daa’imah, 5/337 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: 

If the imam recites in prayer what he can of the Qur’aan, then he forgets the end of the verse, and none of the worshippers behind him knows it, so they cannot prompt him, should he say takbeer and bow, or should he recite another soorah? 

He replied: 

He has the choice: if he wishes he may end the recitation and say takbeer or if he wishes he may recite a verse or verses from another soorah, so long as that is not in al-Faatihah. In the case of al-Faatihah, it is essential to recite it in full, because reciting it is one of the pillars or essential parts of the prayer. End quote. 

Majmoo‘ Fataawa Ibn Baaz, 12/129 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 

If I am praying on my own and I make a mistake in the recitation of the verse, and I am not able to complete it and I am confusing it with another verse, what should I do when I am praying? 

He replied: You can do one of two things: you can either move on to the verse that follows it, or you can bow, because the matter in this case is broad in scope. End quote. 

Fataawa Noor ‘ala al-Darb by Ibn ‘Uthaymeen, 24/141 

And Allah knows best.