How can I stay motivated without concrete, guaranteed benifits?

Find a real world dream job opening. Write down the dream portfolio that would get you that job. Create every one of those pieces. Each such piece will directly move you forward towards serious money and satisfaction. You are being paid for the portfolio: it is your very first assignment at your dream job.

Would that people would consistently pay me for structured play. That isn't really how it works. Unless you're super lucky, you generally have to pay to learn (often in time and money) and not the other way around. Particularly in a field as competitive as the arts. As an artist, you can make your own work and not be guaranteed to sell it, or you can work as a contract/employee/commissioned artist and you won't get the "directed learning" that you want most of the time - but you will get paid. Most artists I know do some combination of all of the above. To do freelance work, you need contacts and you need a portfolio. If you don't have enough to get clients, you'll need to build one and network your ass off. And working for yourself as a freelance artist is always going to be somewhat hit and miss; that's a big reason a lot of artists like to find steady employment as an educator/designer/animator/museum worker/etc. It's not that it's a moral judgment that artists should only make art out of love, it's that it's a pretty saturated field that's often undervalued and it's hard to make a sustainable living at it.

I have similar problems. Somewhat counter-intuitively, I find motivation in having paid for something. For example, if I pay for a class, that motivates me to attend the class. You could use this to pay for some sort of class during which you could work on your portfolio. Another good motivator is social commitments. Telling a friend I'll work out with them Saturday morning and knowing that they'll be disappointed if I don't show pretty much guarantees that I'll be up at a horrendous hour. You could find an art buddy to work with. You don't have to actually do work with each other, just show up in the same place at the same time to work on separate projects. Finally, I want to point out that there is no “should.” You aren't “happy to study in [your] spare time,” and that's unlikely to change, so don't think you should be any way you aren't. It sounds like you're already coming to terms with this, but it's one of those insidious things that most of us (me) need reminders about from time to time.

Very few of us are naturally self-motivated. Some structure helps. There's a fair amount of research that shows if you externalize your goals by - making them specific - announcing them publicly - and putting your money where you mouth is (you pay out a cash penalty if you don't do what you committed to) then you greatly increase your chances of reaching them. There're lots of tools on the web to help with that. http://www.stickk.com/ seems pretty good.

Illustration: I knew a guy who used to charge a few bucks (about $15 or so, though this was about 30 years ago) for illustrations of RPG characters. I knew another guy who did the same for painting figurines. They were both gamers and it was casual income. Neither made any attempt to develop it into a serious business. I felt like they could have easily done so by attending cons periodically, developing a reputation, bringing some work with them to sell and taking orders for more custom work. What kind of illustrations do you do? Do you have any hobbies? Is there any way you could use a hobby as a pathway to developing a portfolio and/or side income? Is there any way you could expand that toe-hold into a serious side business? And so on and so forth, having each next step grow organically out of the last?

No. You have to keep trying. Determination and grit are what get you through. It's 1% inspiration and 99% perspiration.