Hi,
I agree, try eating more protein than bread. A bagel is heavy to eat and than it swells up. I myself don't do well on breads. The headaches could be lack of protein ... but what about your sugar intake? If you are cutting down on sugar (which you should) you will get headaches for the first few days.
Also, I just had my 1st fill ... 9 weeks post op. Are you sure you need a fill??? I noticed I started eatiing a little more than when I first had it done about 2 weeks before. I use the rule - eat about 1/2 to 1 cup of food. I found myself eating 3/4 to 1 1/4 cups of food. The weight was coming off in oz. instead of by the pound, because of eating more calories and carbs at that point. Since my fill, I am now back to eating a little less than I was when I 1st had the surgery, so about a 1/3 of a small child or senior plate if you go out to eat. I always have take home food for a couple of days :-).
And yes you need to drink lots of water (and no soda). But remember, don't drink before (1hr), during, or after (1hr) eating. It is so important not to wash the food down, it will make you eat more.