You're human, just like everybody else. Everyone gets tired sometimes, everyone gets frustrated sometimes. However, don't let that stop you from trying to get to your goal. Here are some ways to help you maintain a steady pace toward your goal.
Don't strain yourself. If you strain a muscle or cause a stress fracture by pushing yourself too hard, you'll end up undoing all your hard work as you wait for it to heal. Safety first, then work on intensity. If you don't know how to do a particular exercise safely, talk to a personal trainer so that you can be sure not to damage your tendons and joints.
Be a joiner. People don't tend to quit as often if they have friends that they regularly see, so making friends who will also be friendly rivals is a great way to motivate you to show up to every game. It's particularly important that you enjoy the activity, though, so don't choose rugby if what you really enjoy is jogging; you'll make new friends wherever you go.
Make sure you're wearing comfortable clothing that's right for your activity level and the temperature. If you're going to be lifting weights, don't wear clothes that bind at your shoulder; if you're going to be running, don't wear a long, bulky coat or tight pants that bind at the knee. If it's cold, wear lots of layers so that you can easily take them off as you get warmer from exertion.
Make time in your day for a workout. Instead of saying you'll do it when you get home from work, wake up at a fixed time and go for a half-hour jog before work. Or go to the gym a half an hour after you get off from work. The fixed time will remind you that it's something you need to do.
Make sure you allow yourself enough time to warm up, cool down and rest. Going to a high-exertion level immediately or going from a high-exertion level to a quick stop is a sure way to make your muscles cramp up. Stretch beforehand, make sure you rest between repetitions, and stretch afterwards to make sure you get adequate blood flow to your muscles at all times. It will also make you feel less exhausted.
Give yourself a small reward every time you meet an intermediate goal. Make sure your goals are achievable with a little hard work. Maybe when you start, you can buy a new thing for your hobby after you can do 25 push ups in a row. Maybe you can buy yourself a new gadget after you successfully run a mile on the treadmill without stopping. Maybe after burning 400 calories, you get a piece of candy. Whatever works for you is the best way to go.
Don't worry about it if you miss once or twice. In fact, you might build that in-you get to miss once every two weeks, twice a month or whatever. But don't let that turn into an excuse to not do it at all either. Give yourself a mulligan once in a while- missing one day won't kill your routine unless you let it.
Use these simple ways to keep yourself fit by exercising regularly. You'll improve your energy, concentration and mood much more rapidly than you'd expect.