My eye that can see better has a stronger prescription...?

You have a moderate short-sighted prescription with a small amount of cylinder (astigmatism) denoting a little distortion of each eyeball. If I knew a) your age and b) the date of your most recent eye test it would help me to explain possible reasons for your feeling that your left eye is weaker when in fact it is the right eye that is slightly weaker, but only by a small amount. Your eyes are fairly well balanced but one or other will be dominant and may seem to you that it is more 'tired' because it may work a little harder. Also your occupation, whether you are at work or school/college will determine how much you need to wear your glasses. They are probably required full time, but you will be able to read perfectly well without them at normal distance (approximately up to 14 inches) although blur will be evident at any distance beyond that.