The Irony of My Memories!

I have a terrible memory. I forget. Almost religiously.I used to be pretty good with names, but lately that too has gone sour. It's like I have an unbelievably low capacity when it comes to memory retention.
In the middle of a story, a story I'm telling, occasionally I stop mid-sentence. I get this odd puzzled expression on my face. I'm surprised at myself. I have absolutely no idea what I wanted to say next, or how the story goes. I'd be talking on the phone and minutes after we hang up, I try to recall what we agreed on, and sometimes I go blank. For the life of me, I don't remember if we decided to meet at 7 or 7.30, if I was supposed to pass by or meet them there, if I had to pick something up on the way or if the restaurant was the next right, or left for that matter.
It's sad I know.
It also happens with my reading. I'd be reading a novel, and then it comes up in conversation and I don't remember much of the details. Details I'd just been reading. Even in my exams, I face the exact hopeless dilemma. Like which management theory first stated motivating employees, or what was the correct term used to depict a certain state of things. It wasn't lack of studying. I concentrated hard enough. Maths was an exception though. I think I just memorized equations by heart and it worked. A tiny miracle I must say.

So why is it that I can not forget things I really really no longer wish to remember? Some memories of times long gone, a different day and age and a different life altogether. I do remember those. Not vividly, but it's all there. Plenty of bits and pieces that jump right at you. It's not nostalgia. I'm not sure if these memories are simply haunting me, or they just linger. My point is I easily forget. Exceptionally easy. So why is it so damn hard to forget when you put your mind to it?