It is probably not getting enough light - that makes plants to etiolate, grow skinny & weak. They can't support themself...On top of that it may be overwatered, or in poorly draining (water retaining) potting mix - what is it made of? and is there a drainage hole in the container?

Beautiful cabinet! Agree, not enough light. That spot would be better suited to a Dracaena, snake plant (Sansevieria), parlor palm, or Pothos on a pole.

My experience is that once they bend over, they won't recover back to vertical. Have had this happen twice (not from lack of sun though) and had to start again, cutting up the plant and using it for cuttings.

A lack of calcium might also cause that. I think Al wrote something about that.

tj

It's a Euphorbia, not a cactus - be careful of its milky sap, oozing out of cuts / damage - the sap's properties are such that it can cause caustic burns on skin and is particularly painful in any mucous membrane.

Your plant is not a Cactus, it’s an Euphorbium, like all plants it needs water to survive. Seeing the browning leaves, the extra dry earth it grows in, it is dying of thirst!

You have to water it well in the summer, about once a week, while making sure that it will dry out completely in about 2-3 days as Euphorbias do not like to keep their feet wet for prolonged periods, but still need to drink. You should put it outside in the summer, in a shady area first, then slowly get it used to a place with at least 2-3 hours of morning sunshine, while watering it when it dries out (it could be sooner than 2-3 days). When you bring it inside for winter, reduce watering to a good splash, always depending on the warmth of your house, and put it close to a sunny window where it will get at least 2-3 hours of sunlight or a minimum of very bright light.

Your Milk Tree comes from Africa where it gets pounded by sunlight from all sides! Living in the ground makes it crave the warmth and light of direct sun, but keeping it restrained to a small pot reduces its cravings. You have to make sure the pot is well (very well) drained and no standing water remains once you watered your plant. Get the proper earth mixture for Euphorbia as it needs to be in a very well draining soil.

When I look closely at the soil in your pot, it has the white build up made by watering with hard water. If that’s the problem, try to gather rainwater, aquarium water, melted snow to water your euphorbium, it will feel a lot better. No plant likes hard water, it’s too alkaline for their well being

The bending over could also be that your plant is lacking silica (or silicium, sorry my english is lacking the right word), you should inquire at your Garden Center where to get the supplement and how to use it. All your plants would benefit from it. Silicium is responsible for cell strength and growth, a good gardener should be able to give you the required quantities and the right product to use. Here is a recipe for nettle and horsehair tea that could help your plant absorb the required silicium. Make sure you use rainwater to make it as your water seems hard. You can use one or the other or both of the herbs, easily found in a herbal shop. Water your plant with the mixture instead of plain water, it should help. https://gardender.com/nettle-manure-and-field-horsetail-tea/

I agree with everyone who says your plant needs more light, but I doubt your problem is only from lack of light. If it were so, your plant would be twisting its branches towards whatever light it gets and not fall over like a weeping willow. Euphorbias are plants that grow in full sun in their natural environment and if you want your plant to be happy, you should strive to give it an environment similar to the one it originates from.

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Let us know how you did! Good luck!