When something good happens, do you ever feel it's a trap?

Yes, sometimes I feel something bad will happen, but sometimes it’s because I think I haven’t done something correctly. 

There was an occasion a few months ago in which I was sure something really bad would happen and it went on for nearly a week. In the end nothing happened.

I know how that feels. I've been let down many times by psychiatrists, so I'm always hyper vigilant of incoming disappointments and letdowns...

Yes often, I wonder if someones having a laugh at my expense and if it's all a massive joke, so I don't know how to handle it all and end up upsetting people because I appear ungrateful or uncaring, or just disbelieving.

In general I find reality checking useful, asking myself how likely is something to happen? It dosen't always work though, it dosen't matter how many times I ask myself the likilyhood of the channel tunnel collapsing when I'm in the middle of it or some other Holywood disaster movie event, practically 0 is the answer, but you still ain't getting me on the thing.

You might find the information and advice here helpful, perhaps along with asking your GP for support. The first article, for example, explains that:

"Because of this high state of anxiety, many autistic people find that their brain goes straight to worst case scenario in a variety of situations. This is known as catastrophising."

NAS Professional Practice - Autism and catastrophising

NAS - Anxiety

Yes I sometimes feel like that. Sometimes if I have a good stretch anxiety wise, there is a voice in my head saying it's just taunting you, something is going to happen and it will all come crashing down.