Hello. I can really relate. I can't tell you how to settle your fears, but I can let you know how I settled my fears.
First of all - is there any immediate danger to being alone? Example: I was threatened by a psychopath, who claimed to have access to military equipment to see through walls (and guns) so I felt on display inside my house every night. If you have a similar problem, my only advice is to sit down in day time and assess the danger (with an expert if you can) and decide if it is too dangerous to stay in your house for now, or if your house is safe. I ended up leaving my home for a period, and did not sleep there for half a year, as my therapist and the police could not guarantee that it was safe to sleep there, given that the man who bothered me had shown violent behavior before.
If there is no such immediate danger, but you still feel fear, then you have to consider whether or not you get enough sleep this way. If not, can you sleep in daytime? Can you sleep at somebody else's home for a while? When too sleep deprived, it is really hard to find inner calm.
You ask for tricks to calm you down. What I found most useful when I finally returned back to my home, was to keep a weapon by my bed. I guess this could be triggering for some, though for me it gave a feeling of safety. I also locked all doors, including my bedroom door, to make it harder for anyone to get close to my bed without waking me up.
If you are asking for mental tricks to calm down the anxiety, then what I did, to even fall asleep, was to go over the dangers of sleeping in my bed, down to statistics of how often psychopaths actually kill or try to kill people they threaten (according to police approximately 10% risk) and how long time had passed since he had last been seen close to my house. After knowing exactly the risk I was in, I calmed down a lot. I can't say I slept much the first year, but at least I could sleep once in a while.
Self-soothing has been really difficult after PTSD, but being sleep deprived makes it worst, so I would first of all make sure you get enough sleep, in whatever way you can. I understand that you would like to find a way to fall asleep, but that may not be so easy just by using mental tricks, thus why I stress the physical safety aspect so much.
When I finally started feeling safe to sleep after about 1-2 years, I used chamomile teas at night to calm my system, exercise in day time and calming meditation music at night to fall asleep. It took years to be back to normal, and still, 4.5 years after the onset of PTSD, I don't always feel safe at home. Again why I stress the external factors. Get sleep. Then heal the trauma. If not you will not have the energy to deal with the anxiety, in my experience.
I'm really sorry I don't have an easy fix for you. But I really relate to your anxiety, and I am really sorry you have to go through this alone. It's very tough.