TRACON Controller reported using wrong call sign when issuing descent instructions resulted in confusion on readbacks and a CFTT event.

Narrative

I descended Aircraft X to 5500ft; and told them to keep their speed up for the sequence. I then thought that I had slowed and descended Aircraft Y to 210kts and 4000ft; but I misspoke and clipped that particular transmission and only had the last syllable and I also said incorrect callsign instead of the correct callsign. I missed the read back from Aircraft X slowing and descending to 4000ft assuming it was Aircraft Y reading back. When Aircraft Y did not slow as expected; I issued a new instruction to them to reduce speed further and then descend to 4000ft. I then noticed that Aircraft X was descending below 5500ft and asked them to verify that they were only descending to 5500ft. They read back that they were descending to 4000ft; so I told Aircraft X to climb and maintain 5500ft. I did not issue the low altitude alert since it had not alarmed and did not alarm. I also questioned Aircraft X when they were at 5200ft; so I did not think that a low altitude alert was necessary at that point. I was on an overtime shift and it was my 6th day towards the end of my shift.Suggestion: I just need to ensure that I say the correct call signs and listen better to read backs.