I'm sorry you're struggling, I know brain weasels can be assholes.
To clarify it is a sex date. I find these times to be the hardest part of all of this.
Do you know why you find sex dates the hardest? What's causing the nightmares?
I'm not good at falling asleep if I don't know when someone will be home. Even as a child I needed to know 'when' to fall asleep. I'm a light sleeper, so I want to know when I'll be startled awake or if I will. I always want to know will they be home in the morning (6am -noon) or that evening (midnight-3am). It makes it so I can sleep. I'm not saying when they should be home, just when they think they will be.
What do you do when your partner is away?
At home DIY spa day if I can. Face mask, hair mask, long hot bath, netflix, mani/pedi. If it's nice out I garden. I tidy the house and get ahead on chores. I read; either a good non-fiction book or I do some studying (either for my continued education or on topics I enjoy). I'll exercise.
How can I work through this anxiety on my own?
One thing I'm trying, especially with my anxious thoughts that are not productive, is the act of acknowledging my feelings, understanding that they are feelings and even though uncomfortable they are allowed, and letting them go.
As cliche as it sounds, practice meditation. There are MANY different types of meditation and not every time does it have to last forever and ever. I do a quiet breathing exercise that lasts between 30 seconds and 2 minutes of just focusing on breathing deeply.
Figure out what about the situation is causing anxiety; this is a big one for me. I have generalized anxiety, so when my baseline for anxiety is not 0, I have to look deep and say "hey, why are my feathers so ruffled?"
My brain weasel, I named her Barbara (sorry anyone with that name), and I also tell her to fuck right off with her silly nonsense. I've worked to figure out which 'voice' is my sadness, and fear, and hurt; and which is there to just make things worse and be mean.
Example: I'm feeling anxiety over the fact a partner may hook up with someone new
'Barb' would feel more like: I'm worried about him being with someone else because you may be forgotteeennnn because you are forgettable.
Fear voice would feel more like: I'm worried about him hooking up with someone because of NRE making him NRE and I get pushed aside and that makes me feel bad.
I'm probably not explaining that well...but Barb gets told she needs to shut up, and fear I talk to as if I wasn't talking to myself. Be kind to yourself.