Method of Salaah

Reciting a surah after surah faatiha before going into ruku

Q: I was praying maghrib. After reciting Surah Faatiha in the first rakah I forgot to add another surah and was going for ruku. I opened my arms but just when I said "Alla-"(of Allah Akbar) I realised my mistake. I once again joined my arms and recited the second surah and did rest of the salaah normally. I did not do sajda sahw. Is my prayer valid? 

Also is sajda sahw valid for all mistakes in salaah or is it invalid if you missed fards of salah?

Keeping the head straight during qiyaam

Q: 

1. Would it be disliked if someone bowed their head till their chin touched or almost touched their upper chest, in the standing or sitting positions of the prayer (while keeping their eyes at the various sunnah positions during the prayer)? If so, for what reason would it be disliked, and when would it not be disliked? 

2. If they did this out of feeling comfortable, would it be disliked?

Acquiring concentration in one's duas and salaah

Q: Please advise how to soften my heart. I recently cannot cry. No matter how afraid I am of my sins and meeting Allah Ta'ala, I cannot cry when making dua. If it's my medication, do I stop it? What if I battle without my medication? At the same time, I'm worried about becoming hard hearted. 

I also cannot concentrate at all in my salaah. How do I become a better Muslim?