MUCH better pictures! (and position! ) Definitely showed a little more... and that you need a bigger seat and shorter flap. :) It looks as though your calf is a little shorter in comparison to your thigh length. The size of your saddle is essentially based on the length of your thigh. So I would agree with Luv2ride that it appears that you're going to need a bigger seat and shorter flap. Whether that means a custom short flap or a brand with a flap that tends to run short, don't know. You're going to have to sit in several because different brands fit differently and you'll just have to see what brand and what model fits you best. I'm 5"2 and used to have a 16.5 that fit me perfectly. Bought a 16.5 in another brand? Way too small. Had to return that saddle and go up to a 17. I rode in a 17.5 the other day? Ridiculously big! I don't know what your budget is but if you're willing to spend a bit you should check out Antares. They offer several different flap lengths. (i think it's 0-5. 0 is quite short) and several different forwardness of flaps. Different narrowness to the twist, etc etc etc. So you can get a very custom saddle to fit your body perfectly. Antaressaddlery.com is their website. Smartpak did just start selling them, but they don't have as many custom options. Good deals do come up on ebay as well.
0426091900 pictures from horses photos on webshots
Looking at this photo, one reason you may be constantly in the back of the saddle is that the saddle is too narrow for your horse and sits too low in back. The saddle should set evely, so the deepest part of the seat is more toward the pommel, not toward the cantle.
The saddle is way too small. I would try a stubben roxane S or VSS AP close contact. 17". It's hard to tell, I also don't know the quality of that saddle you are in. I'm 5'8 and ride in an 18" stubben Tristan special with no knee roll at all. Contrary to recent saddle trends, you don't really need huge knee rolls/blocks for any discipline. They have a tendency to force your leg into a cookie cutter "correct" position that isn't right for everyone. There are 3 used roxanes on ebay right now.
Changing the size of the seat won't fix the problem. A lot of saddles fit differently. I had the same issue as you tho. I'm 5'5 but I have a very short upper leg. I had to custom my saddle with a short flap and add some blocks(oooooh so comfy!).
You go take a look at your local consignment store and see what they hae. Maybe try a couple saddles. There are some great second hand saddles out there.
I have this same problem and I am currently saving up to by a Zaldi by Bt. Crump. Had this one fitted to me by a local tack shop. And would recommend looking into it. I didnt have to have it customized so it must be a pretty standard flap for them. You could also look into getting a saddle where the flaps don't come out so far in the front and are a little straigter. My saddle runs at about 1400 dollars which is quite alot for me but since it will be putting me in the right seat well worth it.
I think a Stubben CS series close contact saddle lile the Artus or Edleweiss CS on a 17 or maybe a 16.5" might fit. (edited that to say might after reading the short thigh comment. I have a long thigh, maybe longer than yours and these saddles are not too forward for me in a seat size 16.5") I ride a 16.5 and I know I am heavier than you. The flap length on those is shorter than most ccs. 13.5" from top of stirrup bar straight down to bottom of flap.
That was 31cm 16.5" Artus, a CS series close contact.
Edleweiss or Edleweiss CS will be easier to find. This one is the 32cm and seat size 16.5"
eBay 260532129192 is a 32cm but 17.5" seat. There is also a dark brown called Ebony. I think 17.5" would be too big. But if the flap is too long on the Edleweiss, it would be pretty easy for a saddler to shorten it b/c the knee pad won't also have to be shortened.