This plant was huge when I first bought it but it had a severe infestation of millipedes and since then has been struggling. I'm also don't think the plant was getting enough light and probably too much water. The soil is not compact and I've already lifted out a section and repotted, which seems to be doing really well on the window sill. Should I repot the remaining stems? As soon as they reach a certain height they always die!
You obviously tried a solution that has worked. You can repeat that same solution with the remainder of the plant, but only if you can not get it light otherwise. If the soil is not compact, you can learn to water correctly, what you are missing is not enough light. If the only way to get more light is put it in a smaller pot now then do it. If you can give the plant more light in its current pot then wait until spring to repot.
You write in the comments your only window is a north window. I think if you want plants in this location you first need to learn to water properly. Plants in the winter go into semi-dormant state, even indoor, due to the lower light levels in winter. During this time you want to give your plants less water. When they are dry water them, but instead of keeping the soil moist like you might in summer, in winter you will wait until the soil is fully dry or the plant starts to wilt whichever comes first.
Buy yourself a pack of bamboo barbecuing skewers. Insert one into each one of your plants. When you think it is time to water, first pull at the skewer. Look at it and feel it. If it is still wet then you wait a few more days then check the skewer. If you want you can use just one skewer, inserting it into the plant for 30 mins then checking it or leave them in the pots at all time. I have a skewer in each one of my pots. That way I can do spot checks all the time, not have to wait 30 mins. Bamboo resist rot, but will break down with time, especially if you keep your soil too wet.
The other thing you should probably invest in are some grow lights. They can just be simple grow light bulbs you put in a lamp by your plant or buy a whole grow light unit. They are constantly coming down in price. I recommend using an LED grow light, they will save you lots of money and can be set to emit a full spectrum for your plant. You do not need strong lights, just something extra for your plants in the winter. Have it on for 12-14 hours a day. If I were you I would also run it in summer, if you only have a north facing window. In summer run it for 16-18hours per day. Even if you can not get a proper grow light almost any fluorescent light bulb will give you just enough to get you through winter, but your plants may still stretch.
The plant is struggling to reach out for enough sunlight, that is why it looks elongated. The best thing you can do is to place it in a bright area but indirect sunlight.
