OK, let's evalute your "balances" here.
I'm going to paste in your post and try to deal with each section "difinitevly"
" I am not exactly sure what info you need so i will set out as much as possible.
I weigh 165 pounds and am 5' 6'' tall.
OK, this makes you a relatively "light weight" sailor. 165Lb. = 74.8 Kg.
For a sailor of your weight, in 15 knots to 25 knots, your Carve 145 is very much
too large (as CC suggests).
For this wind range, on the open sea, a board right around 95-100 liters would seem much more appropriate.
My board is a Carve 145 which came with a stock 52 cm fin, Drake I think. I bought a 44 cm Drake fin as I found the larger fin difficult in choppy conditions.
As CC suggests, it's more likely the board is too big, not necessarily the fin.
The Carve 145 really is at it's best in the 12-16 knot (14-18.5 mph) range, with an 80-90 Kg. sailor.
It's just too wide for someone of your weight to handle easily in much more than 16 knots (18.5 mph).
At more than 16 knots (18.5 mph) up to 25 mph (21.7 knots) a 100 liter or smaller board won't have any trouble getting planing, with your 6.5 m2 rig, you can easily uphaul a 100 liter board at your weight, and the smaller narrower board will go over the chop much more smoothly.
I have a range of sails from an old 5m Tush, a new 5.7m Gaastra Manic, a new 6.5 Gaastra Pilot and a Tush 7.8 Lightning. I have a smaller Pryde carbon boom and Tush 75% mast for the smaller sails and a larger Maui Magic Tsunami alloy boom with 60% carbon Powerex mast for the larger sails.
The Carve 145 really is at it's best with 7.0-8.5 m2 rigs.
Your 5.0/5.7/6.5 rigs really belong on a Carve 99-101 or smaller.
If you want to keep your Carve for when the wind is < 15 mph (13 knots), that's fine, but I'd suggest you add another board to your quiver to work better in your stated conditions with the smaller rigs that you have.
Look for a good used Carve 99 or 101 (maybe a Carve 111, but they sail a bit larger than their volume for lightweight sailors).
Yes, you will have some period of "getting used to the smaller board", but that should take just a few sessions. then you will be on your way on a much more comfortable board.
Changing fins, on a board that's simply too large/wide for the conditions and sailor size is not going to fix things and make them real comfortable.
I sail off the south coast of England near Eastbourne in choppy open water conditions. The wind range varies a lot but I tend to go out between 15/25 mph. (13 knots/21.7 knots).
My problems seem to revolve around the way the board performs with the bigger fin in rougher water. It seems sort of unstable as if the chop is rocking the board because the fin is long and a little out of control once I get going.
You might be right on the verge of "tailwalking" with the larger fin, and your 44 cm will settle the board down a bit, but the board is still simply too big and wide.
Where to you place your mast foot on the Carve 145 (front/middle/back of the mast slot)?
I do not know much about the effects of changing the fin and whether indeed this is as important as I might think, so would be very grateful to hear what you all have to say.
Unfortunately, my answer is not the one you wanted to hear, but your board is simply too big for you (at your weight) in open sea conditions.
In really flat water, you could change the fin size and maybe get a better result, but my experience tells me (I've owned several Carve 145's over the years) that your board is simply not suited to the condtions you are trying to sail it in.
I know you've been struggling with this for quite along time, but this is the first time (that I'm currently aware of) that you've give us "the rest of the picture".
You might have had a much better (easier) time getting through the learning stages on something like a Carve 122 or 111.
Now that you have most of the "basics" mastered, there is no logical reason to stay with the Carve 145 at your weight in your conditions.
Hope this helps,