leg conditioning is another thing you should be doing. leg conditioning is basically improving your legs muscular endurance so they don't get tired as fast. this video has some really good leg conditioning exercises:
it says its a keeper specific video, but the first 2 minutes could apply to any soccer player. the exercises in this video are perfect "leg burners" and they would help your legs stay strong for your entire 2 minute run.
running up hills or stairs is another good way to condition your legs.
Sprints are the best way to get fit for soccer in my experience.
If you can run 15-30 sprints every day, you'll get fit pretty quickly.
A long distance jog (3-4 km) every other day helps too.
For building the stamina, you could do other exercises and sports, it should never only be football.
Increase your strength, focus and moral by adding combat sports in your workout routine. Muay Thigh Bags will surely provide as such.