1) Don't be so hard on yourself: If you miss a workout it's OK... just drop the all or nothing attitude attached to it. Pick it up the next day, not the next week.
2) Create TIME in your schedule: It's your decision to make exercise a priority so block off your workout time every day or every second day and make it about having "you time."
3) You are Tired and had a long day: The hardest part of getting past this is really just getting out the door or putting on your workout clothes. Once you actually start and get that sweat on, you'll feel invigorated and I bet much better than you did before you left.