There are good days in the gym and bad days. Sometimes you will be full of energy and othertimes you will struggle to get going. That is just the way it is and there is nothing you can do about it. I witness these ups and downs all of the time with the clients in my gym and in my own training. It is a constant battle, but I have a few ideas for dealing with it.
First of all, make sure that you have the basics in order. Your training plan should make sense for your level of experience and your specific training goal. In my experience I find that many people have no real plan and no real sense of direction. I can guarantee if you have this problem your training is going to be up and down often. Make sure you have a plan that supports your objective.
Another big thing to pay attention to is recovery. Training correctly is very important, but equally important is adequate recovery. You have to be sure that you are not over-training and that you are doing all the things necessary to recover between workouts. Proper nutrition, plenty of rest, hydration, and limiting stress are all very important. Make sure you have these things in order.
There are plenty of other things that contribute to the ups and downs, but do not worry about every last one of them. Focus on good recovery and learning how to program your training more effectively. Learn from your experience. If something isn't working than try a different approach. Do not overthink. Be honest with yourself. Are you sleeping enough? Eating a healthy diet? Living a stress free life? Recovering completely between workouts? If you can answer yes to all of these you should give yourself a pat on the back. Most people struggle with at least one of these issues and many are challenged by all of them. The key is to not get frustrated about it. Learn to roll with it and focus on doing the best that you can.
No matter how consistent you are, no matter how much attention you pay to recovery, and no matter how perfectly you train; there are going to be ups and downs. It is weird. Everything can be going perfect, but one day you will walk in the gym and all of the weights will feel twice as heavy as they should. It happens to everyone from time to time. Just deal with and do what you can do. Try to determine a cause, but don't stress about it. Shrug it off and try again next workout.
Remember, it is never just one thing. There are many things that contribute to training success. You cannot control everything, so just focus on the things that you can. Your results are not determined by one workout. Training is a lifetime commitment. One workout is just not that big of a deal. The cumulative effect of years of training will determine your overall success, so just try to have more good sessions than bad.
Understand, especially once you get past the novice phase, not every workout is going to be an encore performance. A lot of workouts just need to be completed and checked off the list. They may not always seem significant, but they are. Eventually, they are all add up. Do not get caught up in thinking that every workout is going to be rewarded instantaneously. Many of the biggest rewards from training come much later when you least expect it. Many of the adaptations are so small that you hardly even notice them as they are happening, but you will. All work eventually pays offs.
Also, be sure you do not get overconfident when you start having a little success. I have seen many people enjoy a good novice phase only to get cocky and take the foot off the pedal. What do you think will happen if someone stops training seriously? Their progress comes to a stop. Don't allow yourself to get overconfident. Think of the big picture.
Shrug off failures. Shrug off bad days. Shrug off frustrations. Just get through the down times. Just be patient. They go away and things get brighter. Enjoy the upswings. They are fun. It is great to feel fit, strong, and confident. You feel like a million bucks when things are going well. Enjoy it, but remember that for every up there is a down. Just stay steady and deal with whatever comes your way. Keep working. Remain focused on the task at hand. The things you are training for will probably take a lot longer to accomplish than you expected, so hang in there.