Outside stresses have kept my fasts too short lately as well. The only thing I can suggest is to improve your diet, add as much raw food content to it as you can and wait until the motivation is there for the fast. You need to work out what it is that's stopping you short in your fasting attempts and either deal with that problem or wait it out if you can't do anything about it immediately. For example I can't do anything about the circumstances surrounding my stress so I just have to wait it out and keep up with my raw food eating in the meantime. When the motivation is there again I will be able to fast again.