Beats me why a state machine wouldn't work here?
You mention you have 18 menus:
the first number is the state number. this could be stored as a variable called curr_menu
switch(curr_menu)
{
case 0: //menu1 level 1
if (key == NEXT_LEVEL) curr_menu = 1;
if (key == NEXT_MENU) curr_menu = 3
break;

case 1: //menu 1 level 2
if (key == NEXT_LEVEL) curr_menu = 2;
if (key == NEXT_MENU) curr_menu = 0
break;

case 2: //menu 1 level 3
if (key == NEXT_LEVEL) curr_menu = 2;
if (key == NEXT_MENU) curr_menu = 1
break;


case 3 //menu 2 level 1

}

this could also be implemented in a table:
curr_menu indexes into table[NUM_MENUS][2]

table:
db 1,3 ;next level,next menu
db 2,0 ;next level,next menu
db 2,1 ;next level,next menu
...
....
you could also add pointers to the menu strings and/or code to process the menu

table:
db 1,3 ;next level,next menu
dw menu_str1
dw menu_code1

db 2,0 ;next level,next menu
dw menu_str2
dw menu_code2

db 2,1 ;next level,next menu
dw menu_str3
dw menu_code3

.....
.....

sure just about everybody has done a menu system before - don't be afraid to try your own as everyone wants something slightly different.