Energy in physics takes many forms, that can be interchangeable during interactions. The law of conservation of energy holds for a summation of all the different energy manifestations of the system under study.
if a particle was completely isolated would it lose its existence and turn into pure energy over time ?
No . An isolated particle can have energy as its rest mass, can have kinetic energy if it is moving with respect to some coordinate system. There is no way to transfer its rest mass energy anywhere, unless through interactions ( and then it is not isolated) . An isolated particle is stable if it cannot decay to other particles. The electron for example, or a photon, can only disappear through interactions with other elementary particles.
You are probably confused by the simplified quantum mechanical picture of a charged particle continually exchanging virtual photons with other charged particles in the environment. Virtual photons are a useful calculational tool in getting the probabilities for the interactions of elementary particles. The virtual means that they do not conserve the energy momentum measure of their four vector, they are off mass shell, so energy conservation has no meaning in these exchanges.
In classical electromagnetism the fields of charged particles are stable, they do have an energy density but it is associated with the particle as its charge, and cannot be diminished by interactions. This is an observational fact. The boundary conditions of a problem with many charges will determine the composite fields, and any energy produced or lost comes from another form of energy than the field energy density of the individual electrons; it will probably be kinetic or potential energy to start with.
Macroscopically looking at live organisms, yes they continually exchange energy signals with the environment, energy which they have to replenish from other forms ( food).
Not elementary particles though