To me it doesn't really sound like motivation is your issue. If you had no motivation at all then you wouldn't have installed those engines and posted on this forum. I've been through similar experiences myself and although motivation was a part of it, it was much more a problem with lack of focus and an inability to actually 'do' something. Perhaps not a particularly useful response but in my case getting a diagnosis of (genuine) ADHD at the age of 30 changed almost everything. After making many lifestyle changes and starting stimulant medication, which in itself I am not particularly fond of, my ability to get things done started to improve significantly.
I started programming at the age of 13 with the sole intention of becoming a game developer and thankfully I really clicked with the subject. However things didn't really pan out as I would have liked. As I got older the stresses of modern day life combined with several undiagnosed 'issues' (including various 'xias) really impacted upon my motivation & ability to really achieve anything - after all it was hard enough just keeping the pace let alone succeeding in a field such as game development. There were times when my motivation did rear its head above the fog and inspire me to work on something but most of the time I would just feel spectacularly overwhelmed and end up paralysed with "blank-page syndrome". I made a few tech demos over the years such as a Doom 3 model viewer for the original Xbox (I bought an XDK set off of Ebay) but none of it ever went anywhere and I eventually gave up on my dream. There was a time when I didn't touch a line of code for about 5 or 6 years.
Skipping ahead to today at the age of 32 and I'm now a self-employed software developer and I hope to release my first game or two this year. I don't get as much time as I would like to spend on game design/development as I'm married and have bills to pay but I do put a lot of effort in between contracts. I'd love to work on them in the evenings but anyone who has been on Ritalin will tell you about the awful rebound - after 5pm or so I am completely and utterly useless. But saying that, I'm just happy that my ideas are finally getting worked on. If it takes me a bit longer than the average person to get something out the door then so be it!
When I do get some time to work on games do I still struggle with blank-page paralysis - The motivation was there but the difficulty was breaking through the barrier and actually doing something, though this is a problem I struggle with in every factor of life and not just game development. However I did find that drawing inspiration from my own life experiences REALLY helped a lot, such as past relationships, love & loss, places I've been to, previous pets I've had etc. Once I had that single spark of inspiration it was much easier to put pen to paper and start designing. At this point I should mention that I'm a big fan of documentation before development. I find that if I have something written down then I no longer have to worry about remembering it, helping me to focus on the greater project rather than juggling dozens of individual components in my head at the same time.
The point I'm trying to make is that everyone is wired differently. For some people focus and motivation come naturally whilst for others, such as myself, it has to be consciously endogenous. My suggestion, and feel free to disregard it, would be to not let anyone tell you that you cannot, or should not, do something just because it may not come to you naturally - Good things have come from people who struggled to work on them! I also cannot overestimate the importance of focus and scope. Perhaps trying to figure out what you want to make rather than how to make it may be a good next step? Be realistic though. Don't make the mistake that so many (including myself) have made by trying to make the next big thing right from the get go - Chances are you'll either never get started or fail spectacularly!