Barbara, so nice to hear you're back and all is well.
The sternum movement is normal. You won't hurt anything, as long as you follow the doctors instructions regarding lifting objects. That grinding feeling can occur for quite a while, so don't be alarmed if you still have a little bit of it in a couple of weeks.
Depression is a toughy, but it is very common post-op. Your body has just experienced some major trauma and although the physical part is the most obvious, remember that your emotions and mental state have also been floored.
It takes a while for all the drugs, stresses and trauma to work their way out of the system. Think of it as a pendulum. When you swing it, it goes side to side and takes a while to settle back into a normal resting state. That is what your body is going through right now.
Take things day-by-day. Some days you will feel wonderful - other days you will want to cry all day long. The key is to remember that each day, you are healing. Tomorrow is a new day, so today's depressive state doesn't have to last.
I know I suffered from depression for a long period prior to surgery and during my recovery. It can be very debilitating, but if you do not have a propensity to suffer with it, just try to relax and adjust your expectations. It takes a long time to recover.
Sometimes we feel good for a week and then hit a wall where we feel really tired and crappy for a few days. That's all part of recovery. As long as you can see it for what it is and try to adjust your expectations accordingly, you should fare well.
I'm glad you came through everything with such a great attitude. It was scary, but you did it!
Kev