There is no doubt some situation will arise that will throw a wrench in your plans. Suddenly, you get the flu and it sets you back two weeks in your exercise program. Or maybe your job requires you to travel and you can't find a decent place to work out. As a result, accomplishing two workouts a week becomes impossible. Or an old injury prevents you from progressing the way you had hoped.
Setbacks like these are very common. You can't let them halt your progress. Don't look at them as roadblocks, just detours. Based on the circumstances, you need to put together new goals that take into consideration the situation. If it's an injury that limits the amount of weight you can use on a particular strength-training machine, then find a different exercise that doesn't affect the injury. Maybe you can't find a good place to work out while on a business trip. Don't quit-do some crunches, push-ups, or rent an aerobic video.
Plan on things like this happening. Just don't give up. You're only a failure when you decide to quit.