So one possibility is that you’re attempting a novel before you have all the necessary literary tools. You may have a solid beginning idea but not the skill to develop it into enough substance to sustain 100K words, so you’re running out of steam, as it were. Perhaps you don’t know where the story is going and have written yourself into a dead alley, but don’t yet have the critical eye to see where you tied yourself into knots.
Another possibility, as I indicated above, is that a novel isn’t your natural story length. You may be trying to stretch a short story-sized idea over 500 pages. Or you may have insufficient twists and turns and whatever nifty stuff lights you up about writing so that all the fun has gone out of it.
Whatever you do, though, don’t give up. I can’t tell you how many of us who’ve gone on to successful careers writing novels have trunks of unfinished novels. If yours isn’t a source of joy, set it aside. Or steal the juicy bits and weave them together with new! improved! sparkly! bits. Try writing short or short-short. Try poetry. Write journal entries. Blog. Keep at it — and notice when you hit your stride. In other words, go where the fun is. The fun and the heartbreak and the adrenaline. That’s what will sustain you page after page, novel after novel, for an entire career.