Q: MBP wont start. Have used search and tried all suggestions.

Hi, sorry for making a new post about this, but I havent been able to find a solution in my 3 day searching efforts.  I fear my may be out of options.

I'm not to most computer savvy person around, so please hang in there with me.  I know you need some info on my MBP, but I'm not 100% sure and cant get on to find out.  I have a 2 year old 13'' Pro.  I installed OS X Yosemite and am one update behind, as my mac started to crap out and wouldnt update. I will try to make this short.

My mac started running slowly, I noticed it first switch around from safari to itunes to finder and so on.  It was only while it would open.  Once open it would run fine.  This quickly began to creep into overall performance.  Restarting would help for a day or so.  All the sudden 4 days ago I started having issues starting up.  It began by being very slow, but would eventually start up.  I then began getting the folder question mark symbol.  Nothing worked, aside from tapping on the bottom.  Oddly enough that gave me nearly 12 hours of a flawless running machine.  Today I started having trouble again so I restarted.  NOTHING!  Now as I continually try to restart I will randomly alternate between the folder question mark, and blank grey screen and the prohibited symbol.

I have tried starting up hold the option button, to no avail.

The best progress I have made is starting up and holding command - r.  Once in Disk utility everything will run smoothly.  However, nothing seems to help.  I have tried and unsuccessfully to "Verify and Repair disk"  It red is tells me "Repairing volume failed: Disk object invalid or unable to serialize"

This HD says 499.25 GB, only 35.7 MB available.  But there is not possible way that could be true.  I only use that mac for my itunes and browsing the internet.

On the 2nd? or sub? Macintosh HD It is greyed out and says its not mounted.  When I unlock and mount it, it does so "successfully"  I put this in quotes because it I click out of it, it automatically unmounts.  When I attempt to verify or repair it sits on "checking catalog file." for hours.  From memory, when I first got in to recovery, it was mounted and have 300 something BG available.  When I attempted to repair, that it when it became "not mounted"

Also, I attempted to repair "Mac OS X Base System"  And it sat idle for nearly an hour with no progress, so I abandoned that.

I would prefer to fix this myself and to avoid the Apple store.  But I feel my HD may be totally dead and need replacing.

Like I said, I am not very computer savvy, so please attempt to explain as if you were trying to explain this to and elderly person.  Thank you for your help!