What is the ruling on someone who perform their prayer after finishing it because they believe they missed a Rak'a, a prostration, or that they did not perform it correctly (thinking their prayer was invalid)?
If he was sure that his prayer is void then reperforming it is an obligation along with figuring out the reason of invalidity so long as this wasn't out of uncertainty. And Allah Knows Best.
What should a worshiper who has forgotten a pillar of prayer do?
If he/she remembered the forgotten pillar before offering it in the following Rakah (unit of prayer), then he/she should offer it, and continue the prayer, then offer Sujood As-Sahw (Prostration of forgetfulness) at the end of it. But, if he/she remembered that forgotten pillar after having offered it in the following Rakah, then he/she should offer a new Rakah in its (the one in which he forgot a pillar) place, and offer Sujood As-Sahw.
I went to the health center today to take an intramuscular injection of Neurorubin B12. The nurse told me that this type of injection breaks the fast because it nourishes the body. Is this true? I have heard that intramuscular injections do not break the fast, but I was unsure about this type, so I decided not to take the injection.
Therapeutic injections administered subcutaneously or intramuscularly do not invalidate the fast because they do not enter the digestive tract through an open passage. However, intravenous injections containing nutritional fluids do break the fast, as they are akin to food and drink in effect. The B12 injection is a therapeutic injection and does not break the fast. And Allah Knows Best.
I suffer from absentmindedness and constant worries during prayer, what should I do?
You should take refuge in Allah from the whispers of the devil and life worries. You should also seek Allah`s help and offer a lot of Istigfar (seeking the forgiveness of Allah). And Allah Knows Best.