First the good news: you're not alone in this by any means. It happens because we want a painting to come out right; the fear that the result will not be what we imagine paralyzes us. It's also been a while since you've painted, so your skills are rusty; what you're doing now doesn't match what you know you're capable of and that's also inhibiting. It's also because we see our paintings as part of ourselves, not as product; we invest a piece of ourselves in them and want them all to be perfect, which is never going to be possible.
I thoroughly recommend buying a little book called Art and Fear. It's only 134 pages, but I find it wonderful for getting me through moments when I worry that I'm just wasting paint. It's not full of psychobabble, just straightforward acknowledgment of issues, and advice. For instance:
"The development of an imagined piece into an actual piece is a progression of decreasing possibilities, as each step in execution reduces further options by converting one -- and only one-- possibility into a reality. ... the piece you make is always one step removed from what you imagined... after all, your imagination is free to race a hundred works ahead, conceiving pieces you could and perhaps should and maybe one day will execute -- but not today, not in the piece at hand. All you can work on today is directly in front of you. ... art materials seduce us with their potential."
In terms of practical things to do, don't set out to do a finished, complete painting. Do small studies, say an apple and its shadow, a vase, a hand, or an eye. Something that can go wrong or be reworked many times because it's just a learning piece, not a 'real' painting.
With drawing you could take a piece of charcoal and cover the whole sheet, then rub it in with your hand. Then draw back in with a plastic eraser. By doing this you've already mucked up the whole sheet of paper before you start, so anything you do can only be an improvement. (It's a method I often use; for example with this this tonal figure study done in charcoal.)
If you've got some paintings you've done that you like, or like parts of, put these on display to remind yourself that you can do it. If you've done it once, you'll be able to do it again. Painting requires perseverance not talent.
I think my favorite solution is to mess up the canvas before I've even started. To deliberately paint a jumble of color and brush strokes, to show the canvas who's the boss, then wait for it to dry, and paint over this. Everything I then do can only be an improvement. This approach only works if you're using mostly opaque colors, though, which will hide the lower layers! Or if you can't face doing this, stain the canvas with a color as an underlayer, so it's not white. If it's not too dark, you can use watercolor on top too.
Let's give Vincent van Gogh the last word on the issue of intimidating blank canvases: "You do not know how paralyzing that staring of a blank canvas is. It says to the painter, you can't do anything ... Many painters are afraid of the blank canvas, but the blank canvas is afraid of the really passionate painter who is daring."