WARNING: technical post.
TLDR to answer your question: Memantine didn't help me, while clonazepam / phenibut do. Clonazepam lowers the excitation from many different sources, while memantine lowers excitation from a single source which may or may not be overactive.
Clonazepam doesn't work on NMDA receptors. Clonazepam increases negatively charged chlorine ions entering into a cell, and NMDA antagonism prevents positively charged calcium from entering the cell.
However, NMDA antagonists don't lower positive ions from other sources. For example, a lot of excitatory activity in CFS patients is because cell integrity is weak because ATP(energy) isn't always available as fuel for the ion pumps in the cells.
For more info on that, check out here:
https://en.wikipedia.org/wiki/Mechanism_of_anoxic_depolarization_in_the_brain
The less positively charged a cell is, the less excitable it is, and the less symptoms of anxiety, twitching, etc you will get. It takes a lot of energy to prime cells back to their original state after they fire, so the less excitable cells are, the less energy you will use. No depletion of energy = cell walls stay stable. You can see why depressants such as clonazepam are pretty good for some cfs patients.
The question is whether the NMDA channel is a significant source of harmful excitatory activity in your brain.
Personally memantine didn't work for me until 50 mg doses, but at that point the drug can have long term side effects. It started to give me thinking problems, which is totally not worth it. Since I needed high doses, that means that the NMDA receptor wasn't a significant source of excitation. How do I know this? Well memantine has a huge effect on very active NMDA receptors, but very little if the receptor isn't hugely stimulated. Since I didn't get a response from a small dose, that means my receptors aren't so stimulated. The more stimulated your NMDA receptors are, the more effective low doses of memantine are.
Keep in mind people that memantine has like a 100 hour half life, so if you dose 20mg a few days in a row, you will get the effects of like 50 mg. You have to start at like 5 mg for this drug and work your way up.