Loss of Confidant and routine

It also turns out that getting back into shape is a LOT easier the second time around.

There's also a lot of scientific evidence that at least for muscle growth, it's easier to re-grow as the muscle fibers are already setup to grow at these locations (since they have grown there before) so they can just re-synthesize muscle fibers at these locations.

This has an evolutionary backing too. You can see why our ancestors would find this advantageous in famine / feast cycles.

That's unfair as they clearly hadn't stopped to consider that there may have been a perfectly reasonable explanation (which there was) as to why you hadn't answered their call and had automatically jumped to the wrong conclusion instead.

All my fault, apparently, I didn't answer a phone call from them, I didn't know I had a call from them, and then I had the message. 

Have you asked why they feel you let them down?

Blaming me for it, saying I let them down, but I don't know how I did that.

Maybe life just got in the way, or maybe they felt like they needed a bit of breathing space. I'm inclined to think that the fact they have resumed contact means that they still want to know you. If you still want to be friends with them, maybe message them back with a short message along the lines of, "Great to hear from you. Hope you're keeping well".

Not yet

Had they offered any explanation as to why it had taken them a fortnight to get in touch?

So the person has messaged me after 2 weeks of not speaking to me, but I don't know what to do, they were someone I confided in and someone I trusted in, and this has caused me not to trust them and I feel they will do it again.

I have this for working out. On a great day or high energy I do my full set. On a day I'm tired but I know I want to do some exercise, I do a shorter workout. A full work out might be 5 rounds with weights but low energy work out is 3 rounds and no weights.

I ask myself the question what is enough to feel I've completed something. I use this in the day too, low energy routine might be make my bed and put away laundry. Gives a sense of accomplishment

Story of my life.

But, alas, their loss.

I used to devour online buddy-lists. Now I have a select few friends, and feel more connected.

If they don't, then I think it may be a matter of having to put them in your past.

A good many years ago I had a friend from my school days that I had lost touch with. One day, she suddenly contacted me out of the blue. We rekindled our friendship and were close friends for a good couple of years. We kept in touch several times a week via e-mail, and would occasionally meet up. Then, without any warning whatsoever, she just vanished from my life. No explanation whatsoever. I made several attempts to contact her (e-mails, letters, birthday/Christmas cards, etc), but received no response back. Eventually, I stopped trying. This was about 10 years ago, and I've heard absolutely nothing from her since.

And if they don't?

It could be that something unexpected has happened in their life that they need to devote their time and attention to.

As hard as it may be, my advice would be to give it a few days and wait to see if they resume contact, and to try not to worry about it too much in the meantime. Yes, I know, this is something that is probably easier said than done.

They won't talk to me, I have tried, but they just won't respond.

Have you tried contacting the other person and asking if you have unintentionally said something that has upset/offended them?

I understand that boomerang. Quite often when something is troubling me, I will try to engage in activities to distract my thoughts, but it's like the thing that is troubling me is always there lurking in my mind until it's resolved.

I don't know what to do, I have tried taking my mind off of it but it keeps coming back, like a boomerang.

It can be a horrible situation to find oneself in and confusing for autistic people like ourselves. If we have grown accustomed to having someone in our life that we feel we can regularly confide in, and then it suddenly grinds to a halt without any warning, it can automatically cause us to question if we are somehow at fault. It's a situation I have found myself in on numerous occasions, so I can relate.