Custard curdling/splitting

It sounds like your custard was overcooked, causing the proteins to break down. This can easily happen when you make custard in a pan. By the time you realize it's happening it's too late. Try using a double boiler instead, or improvise one using a pot with water and a bowl.

Also, don't cook it too long, it should still be somewhat runny when hot. If you cook until it gets really thick that's too long, cook it only long enough that you see a change in consistency.

If it does start to curdle pour it out into something else right away, and use a stick blender on it, that may save it.

Here's a good page that gives examples of typical custard problems and how to avoid them.

The most common cause for curdling is the wrong temperature. At no point should you heat the custard to over 87 Celsius, and due to heat inertia, and for a generally better texture, you should stop heating earlier. I have found 83 Celsius to be an optimal target temperature for my taste.

If you preheat the cream (for example you are dissolving caramel in it or similar), let it cool down before mixing with the egg yolks. And after the mixing, when you put it back on the stove, wait for it to reach 83 Celsius (or a bit below, if it's heating up quickly), then immediately take off the heat.

You should also start with room temperature yolks (the curdling occurs at lower temperatures if the eggs are heated too quickly), and never reduce the amount of sugar in a recipe (sugar interferes with curdling).

Acids can also curdle a custard, so if you were using a recipe on the verge of too much acid (e.g. for making a lemon tart), the problem could have been that you have not been precise enough in measurement. But if you tried 9 standard custard recipes, the chance that it is curdled by acid is very low.