Oh hun, it sounds like you are having a really tough time

Your post didn't mention whether you taking antibiotics - I assume you have seen a doctor, but if not you need to get treated as soon as possible.

Also, mastitis usually doesn't reduce supply - are you exclusively expressing? If so, have you been expressing as often since you've had the mastitis? Blocked ducts can prevent the milk from flowing well so it can look like a drop in supply. If there is milk there, it is really important to clear it. Have you tried hand expressing under a warm shower, or putting a warm compress on your breast before expressing? Warmth can help the milk to flow. Some gentle breast compression can help too.

My first DS had a tongue-tie and couldn't attach either except with shields. We never looked back after having it snipped - I hope there is an answer like that for you after all your pain and hard work