I seem to collected every star: All the main stars/ secret stars/100 coin..

There are 7 Stars per Course (6 + 1 for 100 Coins)...
There are 15 Courses, so that's 105 Stars (= 7 x 15)...
***There are 15 Secret Stars (these include Toad's)...
105 + 15 = 120!!!
You probably need 2 Secret Stars...
Check your File Info/Data after the Title Screen (it tells you everything about your number of Stars)...
Good Luck!!!

You probably don't have all the secret stars. There are 15 of them. To get the other two, you have to catch the yellow rabbit in the basement, which is the one a lot of people seem to miss. You catch him twice; once before you have 50 stars, and once after. Unfortunately, if you don't catch him once before you have 50 stars, then you can only get a total of 119 stars in the game.
If you did catch the rabbit, then make sure you have all the following stars:
3 from the Toads around the castle
2 from the Princess's Secret Slide
1 from the Secret Aquarium in the Jolly Roger Bay room
3 from the Red Coins in the Switch levels
3 from the Red Coins in the Bowser levels
2 from the yellow rabbit
1 from the Wing Cap Challenge in the nook across from Rainbow Ride

To get the cannon to open, get all 120 stars.

There are 15 secret stars.
1&2.first floor upstairs to the right of the big door there is a room. Go in and jump into one of the princessess pictures. Beat it once for one and I think in under 30 sec for another.
3,4,&5.there are 3 toads in the game that will give you a star for free.
6,7,&8.grab all 8 red coins in the cap places.
9.go into the room with the sunkenship picture and turn right. Jump into the black thing up above and go through. Grab all the red coins by swimming to get them.
10.in the room with the clock turn right and jump into the little thing and fly to all the red coins.
11,12,&13.get all the red coins in the bowser places.
14&15.in the basement there is a bunny. Grab him for a star. (he is fast)
After a while go back and grab the bunny again for the second.

If you did all these then I have wasted mine and you're time

I'll try 2 help u out. This is a star that most people don't know about. There are 2 stars on the Princess's Secret Slide. One of them is obvious, but the second one, not so much. U have 2 make it down the slide in less than 21 seconds. Sounds hard (and yes it is) but I can make it simple.
This is what u do. Run down the slide and as you're sliding, stay on the right side of the wall, if it works, you'll bumb into the wall as it starts 2 turn left, then you'll start sliding backwards. When the walls end, before u start sliding on the steep parts of the slide, jump. If it works, you'll be standing on the slide. Then quickly run towards the barricade that keep u on the slide on the left-hand side of the screen going towards u, (assuming u didn't mess with the camera angle when you stopped sliding), get on the barricade, and do either a long jump or jump and push B 2 do a dive 2 make it 2 the structure below. Then quickly get up (if u hurt yourself) and then run towards where u can see the slide again, jump down, and slide down. If u make it in less than 21 seconds, a star will appear as soon as u cross the finish line. Get that star and u will have 120 stars. (If u didn't already get that one.) This sounds hard, but you'll eventually get it. I don't know what other star u could be missing, however. Try talking 2 all the Toads or catching the rabbit in the basement. Hopefully this helps, and if it does work and u get 120 stars, go 2 the last level and beat Bowser in the sky again and he will say something different.

This is easy. Use cheats. And you will get the cannon opened. But you can't then use the vanish cap