Supplications

This is not permissible.

Simply by repenting ,having remorse and intend not to do again that sin.

You should repeat: la ilaha illa anta subhanaka inni konto mina athalimeen

The Sunnah is to read it only at night.

No, because this is not proper Arabic. It is best to stick with what our Prophet salla Allahu alaihi wa sallam had taught us.

No, this is not permissible.

Yes, you can.

The Prophet salla Allahu alaihi wa sallam said: whoever says this when going to bed, Allah will forgive his sins even if they were as the foam of the sea: lailah illa Allah wahdahu la shareeka lahu, lahul Mulku walahul hamdu wa howa ala kulli shai’in Qadeer, lahawla wala quwata illah billah al aliyil atheem, […]

It was narrated from Anas (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) used to say: “Allaahumma inni a’oodhu bika min al-hammi wa’l-hazani wa’l’ajzi wa’l-kasali wa’l-jubni wa’l-bukhli wa dala’ il-dayn wa ghalbat al-rijaal (O Allah, I seek refuge with You from worry, grief, incapacity, laziness, cowardice, […]

Both are from the Sunnah.