Prednisolone can have some unpleasant side effects. Yes, it can cause weight gain, because it stimulates your appetite, but, as you're not exercising so much (otherwise you wouldn't be on the preds), it tends to turn to fat. This can get into a cycle, because preds also mess with your moods, and can make you very grumpy (though normally when coming off rather than on a sustained dose).
If you've put on weight, and your mood is low, and you have a tendency to comfort eat, it can be a nasty cycle to get into.
It is possible to substitute 'I' for 'You' throughout the above. Been there, man. Many years. Just be grateful that you're not taking large doses of steroid during puberty, as it really messes with your hormones.
But the end point is that oral steroids work. Stick with the course, keep a close eye on yourself, try not to worry about any temporary weight gain which you may be able to offset later when your health is more stable. And things will improve.