Without an adequate resting period after a workout, the fact is, you won't see any real gains in muscle mass other than the temporarily increased size from inflammation (study). That's probably the main reason why you're trying "so hard" but can't see the muscles growing, aside from not having a correct diet and/or supplementation, or having a really small number of muscle fibers due to genetic factors, or a disease, or a syndrome (or the combination of these), which eventually brings you to the conclusion that you are a hard gainer, although that's probably not the case.
Understanding the mechanism of muscle hypertrophy
As I have written in a previous article (how to build muscles fast, the right way), after you train a muscle, you are going one step back from your goal of increasing its size and strength, because that muscle will have damaged Myofibrils in Myocytes (filament fibers in muscle cells) and it needs enough time (from 3 to 5 days) and enough nutrients to effectively repair and enhance the Myofibrils with more Myofilaments. That repair and enhancement of Myofibrils with more Myofilaments is exactly what gives additional size and strength to the muscle, putting you two steps forward towards your goal. So, by not giving enough time to your muscle to do its job of repairing and enhancing, you won't be able to see any positive results from your workouts, which leads to overtraining, inflammation, and in some cases even lost muscle size and strength.
Remember that, our muscles have a limited number of muscle fibers, which is based on our genetic factors, so we cannot produce new muscle fibers at all, but instead we can only repair them (if applicable) and enhance them with more Myofilaments. In case you destroy a muscle fiber, it will be gone forever. So, try to avoid over-training and injuries.
Advice for positive gains in muscle size & strength
- Split your training by making a weekly schedule of workouts and place different muscle groups for each day of your weekly schedule, for example on Mondays train your chest and abs muscles, on Tuesdays train your back muscles, on Wednesdays your shoulder muscles, on Thursdays your arm and forearm muscles, and on Fridays train your legs.
- Do a good warm-up before starting working out any muscle group. Always.
- Your workouts must be strong enough to signal the need for repair and growth. Train the muscles with enough weight that won't allow you to do more than 10-15 repetitions, for as many sets needed until an exhausted state is achieved.
- Allow at least 3 to 5 days before training the same muscle groups again, that way you let them repair and enhance themselves effectively. In between these days you have to supply them with enough nutrients from a good diet, and supplementation if needed.
- If the muscles feel sore, it means that the process of repairing is not complete, so you'll have to wait for more. If you train sore muscles, the potential of growth diminishes, for you're damaging the Myofibrils before they get the chance of becoming bigger and stronger, which leads to no gains.
The above advice is the best way to see positive results if you are serious about gaining size and strength. Also, keep in mind that the temperature of the environment you're in plays a crucial role in the speed of recovery. The higher the temperature of the environment (close or equal to normal body temperature, not higher), the faster the repair process (study). The lower the temperature, the slower the process. Although the muscle cells/fibers are different than other cells of the body and they do not divide through Mitosis (except the tissues that surround the fibers), the study shows a strong base for the generally optimal temperature for growth.