In the English Grammar Profile, B2 point 165 in the category of MODALITY is defined as:
the fixed expression ‘I must admit’ or ‘you must admit’ to express concession.
agree that something is true, especially unwillingly
‘you’ is much rarer than ‘I’ in this structure.
When we search on iWeb for Ngrams:
1 YOU MUST ADMIT THAT THERE IS A 5
2 YOU MUST ADMIT THAT YOU ARE A 5
1 I MUST ADMIT THAT I WAS A 1142 I MUST ADMIT I WAS A LITTLE 102
3 I MUST ADMIT I WAS A BIT 98
4 I MUST ADMIT THAT I DO N’T 93
5 I MUST ADMIT , HOWEVER , THAT 82
6 I MUST ADMIT , I WAS A 77
7 I MUST ADMIT THAT I AM A 66
8 I MUST ADMIT , THOUGH , THAT 60
9 I MUST ADMIT THAT I DID N’T 59
10 I MUST ADMIT THAT I HAVE NEVER 55
11 I MUST ADMIT THAT I HAVE N’T 54
12 I MUST ADMIT THAT WHEN I FIRST 51
13 I MUST ADMIT THAT I DID NOT 50
14 I MUST ADMIT THAT I HAVE BEEN 50
15 I MUST ADMIT , I AM A 48
16 I MUST ADMIT THAT I AM NOT 47
17 I MUST ADMIT , I DID N’T 44
18 I MUST ADMIT , I DO N’T 44
19 I MUST ADMIT THAT I DO NOT 44
20 I MUST ADMIT THAT I DO NT 44