It's important to tailor your workout to fit your goals. That seems obvious, but almost everyone who starts working out eventually finds their enthusiasm waning as their goals get further and further away. One way to avoid giving up on exercise is to make sure your goals are realistic and that you have a specific plan to reach them.
Repeatedly failing to stick to your goals means that either your goal is out of reach or that you haven't quite figured out what to do to reach it. It helps to have a clear idea of what you want and the basics for what's involved in getting it. Being healthy is probably the most common goal these days and, at it's simplest, involves burning more calories than you eat. If you burn an extra 500 calories per day, you would lose about a pound a week. Spot training doesn't work because your body draws energy from the entire body when you exercise, not just from the area you're exercising.
Being healthy is a common goal, there are plenty of people who have trouble keeping the weight on. In this case, your goal might be gaining muscle which, believe it or not, can be just as difficult as losing weight.
Gaining muscle requires careful attention to your workout and your diet with a focus on eating more calories than you're burning and lifting heavy weights. If you lift weights, you can build muscle but if you want to put on some serious size, that requires serious work, extra calories and commitment.
Training for a race or a sport often requires a different approach than if your goal were to gain muscle. Your main focus should be on whatever you're training for. If you want to run a marathon, the bulk of your training will involve running. If you want to be better at basketball, your training will lean towards high intensity jumps, lateral movement and, of course, playing basketball. Whatever you're training for, you'll usually want to include cross-training. For example, you might lift weights to keep your body strong for running or cross-train with other activities to use your body in a different way and avoid injury.
Being healthy is probably the simplest goal to reach since there are tons of things you could do right this second to be healthy. Drink some water, eat fruit, take a walk, etc. Even a few minutes of exercise has a number of health benefits, some you experience instantly and some that may come over time.