Balancing your life can get very difficult if you work on something in your free-time.

I work as a software engineer on my day job, and I've decided to also work on my game in my free time. I basically had 3 choices:

1. Sleep less
2. Work less
3. Have fun less
   

Let's examine these possibilities:

1. Sleep less: if you don't want to or cannot  experiment with polyphasic sleep schedule, it is quite impossible. I recommend to always sleep enough, and be well rested. You need all your brain power. You can even exercise more: I've invested in a pair of quality running shoes, and I go run whenever I can (usually twice a week). Sittting in front of the computer is not very healthy, and exercise helps you sleep well too. Sometimes I can think about future features during run. It does worth to exercise instead of coding once or twice a week.
2. Work less: without changing job that is quite impossible. Be respectful to your employer. But try to have a tight schedule, and don't work overtime! Now that you have your own project you need every minute. Timebox your workday. I try to be a little bit early too rather than staying later, but whatever your preference is.
3. Have fun less: I've actually chose this one. And for me "have fun less" means "go to pub less" and "drink less". I won't have hangover, and I have more time. Friday is still "after work pub" day, but that one is kind of scheduled now. I didn't cut back on general internet browsing, and games, my other fun activities. I don't turn down occasional invitations either (barbeque, restaurant, etc...).
   

Very important: don't steal your time from your family, partner, or other people who you are close to (including pets). It can be very difficult, but if they understand that this project is very important to you they will understand.

Good luck!

PS.: Don't cut back on hygiene ;-)

If you want to create a multiplatform game, you have numerous choices.

You can create everything from scratch. That does not really feasible for a one-man team, so I won't tell more about this. I don't recommend this. I don't even recommend starting from scratch even if you aim for only one platform.

Another option is Cocos2d-x. I've tried it, it is not too bad. It is a C++ framework basically, but there are graphic tools that you can use. The main method of creating your game is through coding though. It is totally free and open-source. I think it gives great freedom, you can integrate basically anything you want, but I think the development is slower.

Unity3D is not free, but there is a free version. Some advanced features are disabled in that, but you can build to any platform even with the free version. A nice graphic interface, where you can drag-and-drop your features to the editor, and you can run your game inmediately. Once you ready to build to your platform, with a few clicks you can port to anything. Of course you have to take care of the different features for different platforms, but the entity based architecture allows you to enable and disable features with one click. (Example: I've implemented touch controls for my Nibbles game, but I can disable them with one click for the Web or Desktop build, and use the keyboard for control.)

What else would you recommend?