I Needed This Breather

At last, after some sizzling days (You could say "after some hot days" or "after some very hot days"), today we can breathe better (I would personally move "today" to the end of the sentence but that's just what I'd do). It has been possible to walk by street ("on the street" seems to work better for me in this context) in the morning and switch off the air conditioning unit. The house temperature has lowered and we will be able to sleep more comfortable (What you said is fine, you could also say "more comfortably" or "comfotably") at night. However, the weather prediction indicates that the heat will come back soon. It is normal in summer! (You could say "This is normal in the summer!") 

That must be a really bad heat wave. Good luck getting through that and keep up your English studies!

Abs work-out

One hour ago, I did an abs exercise routine. I need to turn my abs in rocks! ... or, that's what I'd like to ... Not only for vanity or self-confidence, but for health reasons: I suffer of (replace of with from)chronic backache. Doctor and quiropractic told me that I need to improve my muscles: abs and back. They would give me a string support for the superior part of my body, and help me to my posture becomes better(I'd put "to help make my posture better.). But I don't expect for (I would omit "for") immediate results: I think this will take at least three months before I can see any change. I hope not to get bored before that!

(Very good writing piece. I don't know the full grammatical rules but it seems like there are very few mistakes. Good luck with your abs and your English!)