I can understand where you come from as I have been in a similar situation myself although I find I am most motivated is when I am working alone whilst also, not making paid commissions. While I cannot bring myself to be motivated enough to work with someone else or, to build a project which I was uninterested in the first place. What I find most motivating is what [my] ideal creation should be and you stated that you’d like to
meaning that is the source of your motivation. You will need to dig deeper to understand what motivates you while if questioning work-efficiency with other users. Your problem might not be motivation but coming up with ideas and adapt to strategies that can quicken the development although that is a borderline to being motivated. I deeply suggest drawing, googling builds, playing multiple city format games but while if you cannot concentrate: music, breaks, food & drinks might be a way to resolve the problem.
I am currently building a large-scale building(s) and my problem is getting things together but not in context of being motivated as I’ve spent days drawing up concepts, googling, modelling, walking around my area, photographs but it is connecting the building that makes it seem real.
Moving away before I go off topic, I do want to point out the positives of being a sole-developer as from my perspective your opinion of being a sole-developer is difficult for you which for many others they’ll agree. Although being a sole-developer offers more flexibility to your freedom as you will have control and time. Time is another positive because while team of developers may work faster – they often have a set date which sole-developers have in the past kept adding on without feeling forced and is fuelled by a system of crowd-funding. The only advice I can give to you as a sole developer is to go at your own pace. Number of times while developing I like to vision perfect buildings but I would often try to take as much time possible to make everything perfect.