I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force holding a subatomic particle together? What keeps mass in it's state? I hope this is not a silly question but I am clueless. Thanks 
This is inevitably going to be an unsatisfactory answer because your question is vastly more complicated than you (probably) realise. I'll attempt an answer in general terms, but you have to appreciate this is a pale shadow of the physics that describes this area.

Anyhow, Einstein was the first to spot that energy and mass were equivalent, and you've no doubt heard of his famous equation $E = mc^2$. These days we write this as:

$$ E^2 = p^2c^2 + m^2c^4 $$

where $p$ is the momentum and $m$ is the rest mass. However relativity does not explain how matter and energy can be interchanged. That had to wait several decades for the development of quantum field theory (QFT for short).

If you have never encountered QFT it will strike you as a very odd way of looking at the world. We are used to thinking of particles like electrons as objects, much like macroscopic objects except smaller and fuzzier. However in QFT there is an electron field that pervades the whole universe, and what we think of as an electron is an excitation in this field. Similarly there is a photon field, and photons are excitations in the photon field. In fact all elementary particles are excitations in their corresponding quantum field.

QFT explains matter-energy conversion because you can, for example, add energy to the electron field to excite it and thereby create an electron. Alternatively an excitation in the electron field, i.e. an electron, can disappear by transferring energy to something else. So, for example, in the Large Hadron Collider two quarks meet with huge kinetic energies and they can transfer some of this energy into excitations of various quantum fields to produce a shower of particles.

But this can't happen in any way you please. QFT gives us the equations to describe how the kinetic energy of particles can excite quantum fields and thereby create matter. This is why, to return to your question, mass can't just keep turning into energy. Quantum field excitations only occur in specific ways described by quantum field theory.

And that I think is about all that can be said at this level.
