Dear Lisa:
I am sorry to hear about your experience in confession. The priest should never be impatient, but priests are human, too. Forgive him.

As to your questions, a good confession is when:

1) you confess all grave sins since your last confession and do not hold any back, 
2) you are contrite, sorry for your sins
3) you resolve to repent of your sins, that is, have a firm purpose of amendment
4) promise to try to avoid the near occasion of sin

When the priest absolves you, you are forgiven, and are to perform whatever penance assigned to you.

Since you do not remember what the penance was for sure, your saying many Hail Marys shows the desire of your heart that you are not trying to avoid the penance. In my opinion, it is a good confession.

One thing you may need to remember, is that we are not required to "explain" our sins. In fact, explanations can be in some people a way to try to justify their sins.

The Church requires that we confess only kind and number, unless the priests asks us for details.

Thus, as an example, let us say that I shoplifted at Pennys, Sears, and Target; that I was uncharitable to a co-worker, to my uncle, and to the next door neighbor, and that I had lustful thoughts about my boss, the next door neighbor, and some girl on the street.

My confession would be as follows:

Bless me Father, for I have sinned. It has been one week since my last confession. In that time I accuse myself of shoplifting three times, that I was uncharitable to three people, and I had lustful imaginations three times. For these and all my other sins which I cannot now remember, I am heartily sorry and firmly resolve never to sin again, and ask of you, Father, pardon, penance, and counsel?. 

The details are not needed at all unless the priest asks for them.

As for going blank before confession, write down the sins in advance while you are home when you do an Examination of Conscience. Then take the list with you when you go to the confessional. Be sure to destroy the list afterwards so no one will read it. Either burn it or tear it into little pieces.

There is no need for fear. When you go to confession next time, you can tell the priest iif you wish that you forgot the penance at your last confession, but that you said whatever number of Hail Marys for your penance. He will surely tell you that you did good. Then go on with your Confession as usual.

If you keep your confession to the simple "Kind and Number" as the Church advises, perhaps the priest will not get impatient.

If you do not know the number of times you committed the sin, then say, "I committed ___ numerous times, not sure of the number. If the priest insists on a number, then make an educated guess.

Be not afraid! Your last Confession sounds like a good one. You just forgot the penance. I have had that happen too. God knows your heart.

God Bless,
Bro. Ignatius Mary