How to make a good homemade Croquette
I have always made croquettes with a filling surrounded by mashed potato. So that would be my first suggestion; another suggestion follows.

Make your filling as normal, allow it to cool. If it is too liquid, just drain extra. You want your filling to be moist but cohesive. NB: drain extra liquid before cooling; the extra moisture can compromise the structural integrity of your final product.

While it is cooling, make very smooth mashed potatoes. Season well, and include one egg yolk per two potatoes (assuming a fairly 'normal' sized potato, say 5 inches long or so; adjust for your local differences). Use only just enough butter and cream to hold the mash together and make it pliable. Chill rapidly.

When all ingredients are chilled, you may form logs:

Lay out a sheet of parchment paper or saran wrap. Spread a 1-1.5cm even layer of mash onto the saran. Lay a long 'sausage' of your filling along the mash, about 2cm from the long edge, parallel to it. Using the saran as a handle, carefully roll the mash around the filling to make an even layer around the filling. Pinch closed. Reserve extra mash for another use. Roll very tightly in the saran wrap and chill until quite solid. 

Or you may form balls:

Take approximately a tangerine-sized amount of potato in your hand. Spread it to an even layer 1-1.5cm thick on your palm, in a rough circle. Place approx 2tbsp of your filling in the centre of the potato, and working around the circle slowly bring the edges up over the filling, pinching closed in the centre. Roll gently in your hands to smooth out any imperfections. Chill until quite solid.

When fully chilled, flour/egg/breadcrumb (I prefer panko, but use whatever you like) your croquettes. If you are making balls, I've found that a superior method is to deep fry briefly at 350 until barely golden. Remove from the fryer and allow to sit for 2-3 minutes (this does, we tested, allow the warmth to penetrate through). Then fry again until golden brown, consume with whatever condiments you prefer.

So that's one way.

For your specific recipe, there's a few options. If you want to really have fun, form your croquettes, dip in liquid nitrogen, then bread and fry as normal. Or you can just do this in your freezer. 

But I think your problem is really how you are making your filling. I think your best bet would be to slow cook your meat with very little liquid--a long slow braise would be ideal, think of something like pulled pork for an example. Use any leftover cooking liquid, reduced, to provide enough moisture to hold your filling together.