If you only exercise when you feel good, there is every chance you may not exercise much at all. With busy schedules, long working hours and endless commitments, a training session may get pushed way down the list and can be missed completely.
When you feel tired before hitting the gym, your mind will give you a whole host of excuses as to why you shouldn't go, "you've had a long day", "you need to get up early", "you didn't eat a very good lunch", "I think my left knee is hurting a little". On some days it feels like a constant battle with your mind just to simply show up. It's human nature to preserve energy and this is why our mind often convinces us not to exercise. However the feeling after is almost always 100% amazing. I have only had the odd workout where i didn't feel much better afterwards in my many years of training.
For the majority of training sessions, you have won half of the battle simply by showing up. However for some, you may get to the gym and feel weak. If you are feeling tired before hand or weak during a workout, don't always see this as a bad thing. If you push through and get to the end, this will strengthen your mindset. You will feel great about finishing the session when your mind told you that you couldn't. When you encounter these feelings the next time, it won't feel alien to you. You have been here before and you got it done. You can do that again. You will miss less training sessions and you will start to get closer to your goals.
Remember the saying "Discipline is the difference between what you want now and what you want most". Yes, after work we would all like to go home, switch on the t.v and order a takeaway. That's what you want right now. However is that going to get you what you want most? No. If what you want most is a flat stomach, to be fitter for the weekends game of sport, to build muscle, to lower your blood pressure and improve your health, then you must focus on this. Think of why you're going.
Sometimes we have a tough day. Don't let this stop you, if anything let it drive you to get it done. The tougher your mindset the stronger you will become when pushing through obstacles and getting what you want.