Why do I keep blowing the ceramic fuse

First thing to do is check the door switches. They're set up to sequence the opening and closing of the door with the electronics in the microwave and it's quite common for them to fail, resulting in the door switches being in a state the microwave is not expecting; for instance one switch saying the door is closed while another one says it's open.

There are usually three switches, and they'll usually be a mix of normally open and normally closed switches. You can frequently tell if they've failed by looking at them and manually operating them, but not always, so it's usually best to test them with an ohmmeter or continuity tester. Unplug the microwave and remove the cover; you can normally find the switches fairly easily near the door latch. Disconnect the wires going to them and hook up a continuity tester then open and close the door and verify the switch changes state from open to closed or vice versa, depending on the switch.

Be careful of the capacitors near the microwave tube; they can hold a lot of voltage; enough to cause serious harm so stay away from those.

If you're not comfortable working around potential high voltage sources, you might consider taking it to a shop instead. I've personally worked on several of my microwaves and found that 90% of the time when I have an issue with them it's usually because of the door switches.

Good luck; let us know what you find.