Just my 2 cents:

Try this drill to get to 50m UW. I did this my first try, and the idea is to build up to it in steps to make it mentally less challenging. This method may also have some physiological advantage, but all I know is it's much easier than just hopping in cold and going for a 50 UW.

400 warm up:
-freestyle (200)
-backstroke (100)
-breaststroke (100)

6x50 UW: 3 breaths, 2 breaths, 1 breath, 2 breaths, 1 breath, 0 breaths.

Remember to go slow and think about propelling yourself the maximum distance with each pull/kick, which means gliding after each kick before your next pull.

Keeping your head down will do two good things, it will streamline your body, and you will not be discouraged by the remaining distance to the wall if you can't see it.

Finally, stay close to the bottom of the pool. The closer you are the tile/stripes painted on the bottom, the faster you'll perceive your movement through the water. Feeling fast helps keep me calm and motivated.

Regarind the "6x50 UW: 3 breaths, 2 breaths, 1 breath, 2 breaths, 1 breath, 0 breaths"

Are you saying do 1, 50m in 3 breathes than 2 etc.. Just to clarify.

Thanks.

Yes.

I am a big fan of increasing the distance of the underwater on one breath, rather than decreasing the number of breaths. I have always felt that the first four underwaters are the hardest and after that you are warmed up and at your peak. You don't hit a down side again until about 8-10 underwaters and by the time your Instructors take you there, it is well into gut check time and nobody knows what they will do until they are in that moment.
So, my advice is do four underwaters at a semi-challenging interval (not your hardest, not your easiest), rest an extra minute or so, then try a 30M (hit the wall, do a turn, push off and come up at the end of the first stroke/glide). A day or two later, try a 35-40m. Work up to a 50m.
An alternate approach that I favor is doing the four underwaters, resting an extra minute or so and then just going for the 50m right away. If you don't make it, see how far you can go and then try to beat that mark in a day or two. Keep going until you get to 50m.
The latter approach seems to work best for me because it is all or nothing and really is what the 50m is about. Having said that, I never even attempted a 50m underwater until the Cadre made us do it, curiously enough, after four or five 25m underwaters at about a 2:30 interval (not the easiest, not the hardest interval we had faced up to that time). Everybody on the Team made it, too.
See what works for you. If you don't make 50m, don't worry about it. You only have to do it when the Cadre tell you to and if you have a never quit attitude, you will be fine. If you don't have the right attitude at the time, nothing you do beforehand makes any difference anyway.
Good luck.

Completed my first 50m U/W yesterday. Not trying to sound arrogant but it was easier than I thought it was going to be, and probably could have made it a bit farther. Made it across in 4 stroke & kicks each time. I just relaxed and went for it my mind kinda drifted when I was under; Felt great. I had a swimming buddy of course.

Congrats, Funkmaster! 4 strokes is ideal; I've never made it in less than 6 strokes and 8 strokes was pretty common. The really good guys take 4 strokes so you are right in there. This should give you some confidence. Remember it when things get tough and you will be fine.

It always took me 8 strokes and it really helped me when I started to really focus on counting the strokes. I wouldn't even look up for the wall until stroke 6 and then I was pretty much there. There's nothing worse that looking up and seeing the wall waaaay down there and your heart rates bumps up and you start to freak out. Stay calm underwater and focus on counting. This really applies more to underwater intervals but it works for the 50 as well.

Maybe that helps a little. Goodluck!

Well done! I'm right around 4 strokes too. As LetsRoll noted, I find counting the strokes and getting the MOST out of each one is hugely beneficial.