I usually try to stay out of the "girl threads" because the ladies should have a place all their own, but this thread really strikes home for me because I am in the middle of downsizing too.
What has helped me the most is simply pretending I am already on the boat, that means cooking for myself, not jumping into my vehicle and driving to the store every time I need something, getting rid of things I won't have on the boat, and that type of thing. It isn't as easy as you might think, you'll know the first time you turn the air conditioner off and make yourself sit outside on a chair or in front of a fan.
I have also taken up activities that I would not normally be interested in, I have a bicycle to ride for example. Riding a bicycle has really helped me a lot, I have learned a lot about bike maintenance, fixed plenty of flat tires, learned to carry all the tools I need to fix it when I ride further away from home, and the like. Besides biking I also try to camp more, or at least sleep outside more, sometimes I just throw things into the back of the truck and drive somewhere and sleep under the stars just for the fun of it, I have even done this in deep winter with a huge pile of blankets!
Getting rid of actual stuff has been hard for me too, but it is getting easier. Now that I have a method to it, it seems to have taken on a life of its own, and I honestly feel better every time I get rid of something. I was and am overwhelmed with junk and I didn't realize how much of a burden it was until I started doing something about it. Getting rid of things is very liberating, and at some point you start to feel that if you take it to its logical conclusion you will be free to just walk out the front door and wander off into the world and do whatever you want to do without regrets.