Trouble with Beginnings

I finally decided coming here was in order, because I've noticed a consistent problem in my plots/writing. I'm hoping some of you will be able to offer me some advice.

Writing is my passion, and while I have trouble with finishing stories (like many amateur writers) I'm slowly working through it. However, I've noticed a disturbing tendency when developing my stories. I can create great middles, and ends. But I cannot, for the life of me, make good beginnings.

Once, I plotted out about 5/8ths of a novel, having even written chunks. I knew what was going to happen from Point B, to C, to D, it flowed very well, with plenty of delicious conflict. But when I tried figuring out what point A was, and how it reached Point B, it fell so embarrassingly flat that I ended up abandoning the whole concept.

I think its accurate to say I have trouble with build-up in general. Once things reach a certain point, my story is great. But trying to write what comes beforehand ends up coming out of torture for me to write, and likely harder for my readers to read. I know its unreasonable to expect them to power through that slop under a vague promise that it will 'get better'.

I'm coming out about this now, because I'm working on yet another story idea. Its currently in the 'brainstorming' phase, where I come up with scenes, characters, and pull together the various plot and subplots into something delicious. But I just know I'm going to run into trouble again when it comes to writing anything before the 1/4th mark and the first major plot point.

TL;DR Have trouble writing the first quarter or so of a story.