1. First, you are STRONGER than you think you are. Your mind will tell you to stop long before your body needs to stop. If you want a break do two more squats or two more push ups or two more of whatever else you're doing. Take it two at a time. If you need the break after those two then take it. If you just WANTED the break and you dominated those last two reps try two more.
2. Give yourself ten minutes. This is important if you find yourself in that predicament where you just can't find the energy/willpower/wherewithal to put those gym shorts on. Give yourself ten minutes. Tell yourself that if you are still tired after the warm-up and those first few blocks of running or those first few sets of ab work then you're done. A lot of times once you pass that ten minute mark you are in the zone and ready to work. We give a lot of ourselves to other external obligations throughout the day, so just start by giving YOURSELF ten minutes.
3. There's no pressure. This is especially important if you aren't training for anything. This is not a race or a competition. If you miss a work out you aren't going to go on to lose at the Olympics. You are working out for you and no one else. No one else cares if you got that run in or you did an hour of power lifting. This is for you.
4. The burn is good! That burn in your legs when you're at barre class means you are making a change in your body! That burn in your arms and chest when you are doing those push ups means you are getting stronger! You are pushing past your comfort zone. You have to push yourself a little bit to get stronger. You have to feel a little uncomfortable to build your muscle endurance.
5. You don't have to feel like you're about to pass out in order to get a good work out. Not every work has to be an hour. Everyone is busy. Even the people that work out regularly. But even the people that hit the gym every day don't have knock out workouts every day. If you can just get in a 20 minute walk, perfect! If you can just get in some push ups or a plank or some abdominal crunches, great! Just move.
6. Finally, get a work out buddy or join a class. Having someone around that checks up on you and holds you to your responsibilities is a wonderful thing. That person is waiting for you at the 6am barre class and if you don't come they will find you. That work out buddy needs you to be a spotter during their weight room work out. If you aren't there they will find you. Also, it's just nice to have someone to laugh or roll your eyes with when the instructor tells you to do something really challenging. You are in it together and then you get a drink after that work out and talk all about how crazy class was.