Try not to stress, first of all. The full feeling goes away once your body gets used to nursing. It has nothing to do with supply. So don't let that make you think you are no longer producing as much.
Certainly any breastmilk is better than no breastmilk but there is no reason to think, based on what you've written, that there is a supply problem. Babies are much more efficient at getting milk from the breast than a pump is. If you only pump, and just to add a few ounces here and there, your supply is likely to diminish.
Adding formula definitely will decrease supply, especially if you are giving that to him first. This is not my opinion it is a fact. He is filling up on formula and then having a little dessert from you. The amount of milk you produce is based on supply and demand. Your supply will meet the demand (except for the small percentage of women who have problems producing milk, but that probably is not you since it's been fine thus far).
There can be a little lag in the supply when babies go through phases where they are upping their milk needs (5 months is a normal time for this it seems). That's totally normal and if you just stick with it after a few days your body will be producing what he needs. But if you are adding in formula, he is requesting less milk and then your body thinks he is satisfied with the amount it is producing and won't up the production to meet the need.
Another thing to consider is that babies have fussy periods or times when there is so much going on for them that they get impatient nursing because it does take a while and they just want to go go go.
There really are a number of reasons they can act like he is that really don't signal a supply problem. Have you considered calling your local La Leche League leader? I don't know where you are but if you go to www.llli.org, you can find a link to the LLL groups in your area. A leader can be very helpful to helping you figure out what is going on and offering solutions for sorting it out.
Since you are feeling guilty about the formula, you should try to get back to exclusively breastfeeding so you don't have bad feelings about it. Or if it is a case where you cannot produce enough milk, talking through the issue with a LLL leader or lactation consultant will help you figure that out so you won't feel badly about it. Whether you choose to go to formula sooner or not, you need to be happy with the decision. Good luck!