I am surprised to hear you say that you wear your retainers only 8 hrs a day. The way they work is constant pressure to the tooth for long enough will allow the bone to heal around it and secure the tooth so it will not move again. 8 hours a day is not long enough , especially if you are feeling pressure when you put them in (this means your teeth are trying to revert back to original position).

From what you describe, the grooves near the gumline are one of two things, and you already mentioned both.
1) Tooth brush abrasion. People who put too much force when brushing and who use a hard toothbrush (most toothbrushes now are soft or ultrasoft) would wear away their enamel. The grooves tend to be rounded and wider. While this is still possible, the change in toothbrushes has made it unlikely in the last 30 years.

2) Occlusal pressure. When your teeth hit together too hard, you put excessive force on them, which has to be distributed somewhere. I won't get into the physics, but simply put, the point of most force is between the impact spot and the bone level. This point usually is right above the gumline.
Think of bending a hard plastic ruler. You can bend it so far without any damage to the ruler. Put too much force on it and the ruler will crack, usually in the center. Get another ruler, and move your hand half way up one side (this simulates the bone around the tooth). Bend it again and it will crack between your hand and the tip, and not the same place the other ruler broke. This is because your hand (the bone) supported that lower part.

Same thing with teeth, if you put pressure on them, they can flex slightly with no permanent damage. Put too much pressure and eventually the tooth starts to crack. This cracking leads to the enamel (outside layer of your tooth) breaking off and you forming grooves along the gumline. These grooves tend to be more angular and thinner (shaped like a V).

With the history you gave, your dentist is most likely correct that your heavy occlusion (bite) is what is causing the problem. Several things can be done.
1) Fix the source. A dentist can check your bite and adjust how your teeth fit together. This is accomplished by polishing off areas where you are hitting harder than others until your teeth hit fairly evenly. This allows you to balance out the pressure. You can expect 2 visits. One for diagnostic impressions (the dentist takes these, makes a cast, mounts those, and grinds on them until they are back to normal...think of it as a trial run because everyone's bite is different) and a second for doing the same thing on your real teeth.
(I mention this because I doubt you want to go through braces again, and if you were given a retainer, my guess is that your teeth are mostly aligned and just need simple adjustments)
2) Get a nightguard. People do the most clenching and grinding at night when they are asleep. You can get a plastic nightguard made that will act as a buffer between your teeth. You basically grind on the plastic instead of your teeth. Nightguards can be replaced, teeth can't (at least not with real teeth).
3) If you are sensitive in those spots, you can have them filled with composite. This will insulate your teeth from the cold and protect them from bacteria. Composites also bond to teeth, so it will help replace some of the tooth structure that has been lost so that you don't damage your teeth more.