Deterioration of composite resin fillings after only a few weeks/months?

So some quick background, I hadn't been to a dentist in about a decade (I'm 30 if that matters) till this past January. Hadn't had any problems or noticeable cavities till about a year/year and a half ago, when a fair few seemed to pop up all at once, a few on the front of my incisors. Now I admit, I'd slacked off after turning 20 on brushing, just once a day, didn't floss, and had slacked off even more after turning 27 or so due to extreme depression (mixture of forgetting and just not caring). But I've now started getting the cavities filled, the ones visible when I smile with the composite resin; the first cavities done this way were filled about two months ago, the most recent about 3 weeks ago. I noticed some sudden extreme sensitivity to cold in the area of the oldest filling the other day, went to look in the mirror, and I noticed there's the start of a hole in the resin (more of a pit, really, than a true hole, at this point), and also noticed that there's juuuuuust starting to be the barest hint of degradation in the most recent one. And I'd like to know why, and if this is going to turn into a larger problem. Since going back to the dentist, I *have* started to try to be better about my oral care; I'm still not where I'm sure any dentist would like me to be, again due to the depression, but I DO make sure I brush at *least once* a day, like I did in my earlier adulthood. I mean...the math, to me, just doesn't seem to add up, even considering the resin likely isn't as strong as tooth enamel-if it took literal years for the cavities to even start forming when I was brushing once a day, how is it the resin is breaking down so fast when I'm keeping to that once-a-day schedule, AND about half the time actually do what I SHOULD be doing (ie., brushing twice a day AND flossing?) Are my dentists just using substandard resin materials, or....(is that even a thing that's possible?) I'm kinda worried cuz I have a fair number of cavities left to go, some of them to be filled with the composite resin, and if they're going to break down this fast even with proper or mostly proper care.....I mean, the hell am I supposed to do, just not eat??? Some extra info: I haven't taken depression meds in ~5 years (it's my understanding they can cause dry mouth, which as I understand can lead to faster deterioration), I don't grind (as far as I know), don't use mouthwash, and my brushing method is: forward and back along the tops, backs, and fronts of the teeth in that order, then up and down along the fronts to make extra sure I'm getting the gums. In the included photo, the area outlined in blue covers the visible area of the whole filling, the area outlined in green is where I see the deterioration, those spots were nice and smooth and white like the rest of the filling when first done; the left image is the newer filling, the right image is the older filling (~3 weeks old and ~2 months old, respectively).