What am I doing wrong?

it could be a combination of a big front foil and a long fuse + big stab, how long have you been foiling for?

I also had the 1550v2 as my first wingfoil foil. It's a very high lift foil and can cruise at incredibly low speeds but anything above say 16kts its gets increasingly hard to keep it in the water. You also have a 70cm fuse which is not helping in this situation as it provides more leverage to the stabiliser. You're also light, about the same as me, and that also doesn't help when going fast in terms of keeping the foil in the water.

First thing, as above, what if any shims are you running? if you're running none, leave everything in the same position and add 1 blue shim to the tail stabiliser, see how that goes. This is how I used to ride that foil. Still too much lift, try a red shim but note that by adding bigger shims and reducing relative stabiliser angle the whole setup will become more and more pitch unstable.

The 1550v2 is not a foil that can comfortably rip around at 20kts, it's just too high lift and has to be pushed to hard to get to those speeds. If you've come from a longtime windsurfing background this can seem very foreign as windsurfing, particularly slalom, is massively back foot orientated which is basically entirely the opposite to foiling.

"What am I doing wrong?"

Using footstraps.

Well at least IMO, I know ex windsurfers in particular like to have their feet strapped in but it is truly very limiting and completely unnecessary until you want to start jumping.

Whenever I couldn't feel the foil properly it was mounted too far back in the tracks. Try putting it much more forward.

Are We talking about Wing foiling .If so my be your over sized on the hand wing have you balanced the board I would think that it should be around 6 to 8 on the track and at your weight my just start ridding the MA 1250 .

What a stabiliser does, and why you need a smaller one.
You taxi along, front foil is taking off, and climbing, and would continue to climb until it breached, but that doesn't happen because you shift your weight forwards to level the board out and cruise. But, ....let's imagine you forgot to fit a stabiliser.... You shift your weight forwards to level the board out, but the board doesn't fly level, it starts to dive and crashes back to water. This happens because there is no countering effect to bring the front foil out of it's dive. The stabiliser is designed to exert downward force, this force acts through the fuselage to bring the front foil up. ( Out of that 'dive' and again to start climbing, whereupon you apply your weight to counteract that climbing, and get level flight.). And so it goes. Now you are foiling faster, that same stabiliser is pushing harder downwards ( basically it's flying upside down) which is levering the front foil into a steeper angle of attack, which will result in the foil climbing towards breaching as you aren't heavy enough to comfortably oppose the lift. A shorter fuse cuts down the leverage, and perceived lift, but a better choice would be a smaller stabiliser, which will allow you to move the mast forward, and or get your front foot back .

@windsurftom looks like at this point you have enough riding experience that switching to a smaller stab and shorter fuselage worked out well for you. But that was not the only way you could have got a good feel out of your foil. You could have simply tried shimming the stabiliser way sooner.

I am also 72 kg, and while learning, instead of switching to a shorter fuselage and a smaller stab, I started playing with the shimming pretty much from the day one. It was easy for me to feel what needs to be changed, because I had former wind foiling experience.

Here's a clip of just 12th day of wing foiling, with a big 316 cm2 beginner stabiliser and with a very lifty 1350 cm2 front wing (really, that had enormous amount of lift for my light weight), and a long fuselage (70 cm measured from the front of the front wing to the end of the fuselage). Placing that huge stabiliser at an angle that's well matched to my weight and to the speed range that I was after at that time, made this foil super easy and playful, despite of the powerful front wing, and a long fuselage. It allowed moving the mast further forward which improved the stability and turning and everything. The thing is, if a light weight rider would simply take a powerful foil (that would work just fine for a heavier rider), move the mast further back (or step further forward on the board) you would become very disconnected from the foil, the turns and pumping and everything would become stiff and difficult, you would breach a lot when going faster, and that would all hinder the progress a lot.

Nothing much to see in the clip, but just in the very beginning there are some examples of how effortlessly that powerful setup pumps once properly shimmed. It's well worth finding the best tuning for your foil in the very beginning of the learning curve.