I see that you were using your dad's MTB, so I assume that even though the gradient was up to 20%, you had low enough gearing to deal with it. My understanding is that no matter what gear you used, eventually just moving forward became too much and you had to walk and that you were able to climb better in a higher gear than a lower one.

This is all normal for a relatively new and less conditioned rider. You'll get better with time and weight loss. For now, don't worry too much about spinning up hills. There's always a balance between what your legs can do and what your lungs can do. The rule is that if your breathing is sort of OK but your legs are giving out - shift down. If your legs are sort of OK but your breathing can't keep up, shift up. This balance point will shift with increasing fitness, both muscular and aerobic. High aerobic fitness is necessary to spin on climbs. You don't have it yet and it can take years to fully develop.

Try to gradually increase your hours of riding per week, maybe by one hour/week for a while. 6 hours seems to be a minimum for reasonable performance. Over 10 hours, the percentage improvement per added hour drops off quickly. You should notice good results after a couple of months of doing that.

The best way to lose weight is simply to eat a little less at every meal.