I have extreme sensitivity to cold in the area of oldest filling. Why?

Hello doctor,

I had not been to a dentist in about a decade (I am 30 years old) till five months back. I had not had any problems or noticeable cavities till about a year ago, when a fair few seemed to pop up all at once, a few on the front of my incisors. Now I admit, I had slacked off in adulthood on brushing, just once a day did not floss, and had slacked off even more in the last two years due to extreme depression (a mixture of forgetting and just not caring). But I have 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 three weeks ago. I noticed some sudden extreme sensitivity to cold in the area of the oldest filling, went to look in the mirror, and I noticed there is the start of a hole in the resin (more of a pit or dent, really, than a true hole, at this point), and also noticed that there is just starting to be the barest hint of degradation in the most recent one. And I would like to know why. Since going back to the dentist, I have started to try to be better about my oral care, I am still not where I am sure any dentist would like me to be, again due to the depression. But I do make sure I brush at least once. I mean the math, to me, just does not seem to add up, even considering the resin likely is not 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 am keeping to that schedule, and about half the time actually do what I should be doing (that is brushing twice a day and flossing). Are my dentists just using substandard resin materials, or (is that even a thing that is possible?) I am worried because I have a fair number of cavities left to go, some of them to be filled with the composite resin, and if they are going to break down this fast even with proper or most proper care, I mean, the hell am I supposed to do, just not eat?