First off, I applaud you for taking on such a task. Not many people are willing to make such a cimmitment to achive their goals.
I once made a 3 and1/2 year commitment that was similar. It involved working fulltime during the day, teaching adult education 2 nights a week and taking coledge courses 3 nights a week. When things got rough, I just kept focusing on the goal. I wanted to finish my BA in Computer Science before I was 30. I had left school when I was 21 and returned when I was 26. I made it with 3 months to spare. There were a lot of times I wanted to quit, but I just kept telling my self that the only way to my goal was to stay the course. If I let up or took a break, my goal would not be possible and I just wasn't willing to give it up.In the end, the goal was worth the pain.
When it gets rough, remember why you are making the effort. Remember that everything worthwhile comes at a high cost and think about how good you will feel when you succeed. I have never regretted those 3 and 1/2 years. Whatever you are going for, if its important to you, it will be worth the price!