It's got to be tough handling full time job and school. I certainly don't envy you. You're challenge is a tough one, but as I see it, the first decision you made was a great one. You asked for help!
There are so many ways to stay positive about your efforts. One of many ways is to do a search on the internet (Google or Bing) for positive reinforcement quotes and posters. Or access all the motivational quotes on Spark. You will find them under the header Articles & Videos above and then fourth position down on the left column is Motivation. Click on that and you will find pages and pages of motivational posters. You can print them off if you like or if you are on Pinterest, pin them to your boards there. Or just go there every day and read 3 or 4 or 5 or as many as you have time for in your busy day. So many of them seem to speak directly to me. Maybe reading these daily will help you out.
Second, don't get discouraged if you "messed up" today. That's why God gives us tomorrows. You will do better. Just stay positive about your fitness (all you need is 10 minutes a day and with a little creativity, maybe you can find ways to sneak the 10 minutes in while you are working or while you are in school.
Third, read as many of the articles on Spark as you can that are directed toward people like you with busy lives. There are many ways to pack up a healthy lunch, so you don't have to run to a fast food place. There are also helpful hints for keeping healthy snacks at your work station or in your bag or purse that will keep you on track for your nutrition.
Last, but not least, make sure you are getting enough rest. Not getting enough sleep can greatly throw off your success in losing weight. Sleep is one of those things that seem to elude a busy person like yourself, but whenever possible close your eyes for a few minutes and just do some deep breathing to cleanse your mind and your lungs.
Best of luck to you. I will be thinking of you. Stay positive...