Classic beginers mistake is starting with something too skinny.
There are guys here making superb branch and sapling bows, but it's tricky, you want to aim for at least 2" wide IMO to give yourself a sporting chance. But even with half of a 2" branch it's still rather thick, and as you start thinning it, it will become narrower, then you may need to decrown and it all get more complicated. That's why board bows are a good way to start.
Or go for a 4" diameter log and start with a 1/4 of it, it will have a flatter back and can be made thinner without automatically going too narrow.
Make it plenty long too, we all start off thinking those short bows with mad curves and flip tips look great, but if you go for a 40# draw weight 64" or more long and 2" wide at least you have a sporting chance of ending up with a shooter (ok it will prob be 25#
)
Once you've got one shooter, you are on your way.
As for wood species, it depends where you are, in the UK, I generally suggest Hazel or Ash as they are plentiful.
Have a look at my Bowyers Diary (just google it) if you want to see how I do it
and how I fail...
One word of advice... I don't care how slow you go at it slow down! Only work in short bursts and always put it up on the tiller before and after doing anything, that includes having a mug of tea (or coffee) or taking a leak!
If you really want to go the branch route, make the damn thing 72" long, then you'll have a chance.
BTW. At what draw length are the ybreaking? Maybe stop at a shorter draw, get some shootin' out of one of the bows and see how the limbs hold up, or where they stert to chrysal or bend too much, it might give you an idea where you are going wrong. Post pics here as you progress, you'll get helpful criticism as we've all been through the learning process.
Del