We present a novel theory of fundamental particle spin, deriving the energy-frequency relation of quantum theory, the Schwarzschild metric of general relativity, and the fine-structure constant from this single framework. Our theory posits that fundamental particles are composed of fundamental charges (FCs) and derives the energy-frequency relation and spin angular momenta of these particles from the circular motion of their constituent FCs, as described by Newtonian classical mechanics. The conservation of energy and angular momentum necessitates that each fundamental particle possesses wave-like properties, mathematically represented by corresponding wavefunctions in quantum mechanics. The phase angle of the wavefunction directly corresponds to the angle of this circular motion. The energy, which is proportional to the frequency of the FCs' circular motion, is expressed as the result of applying the operator iℏ∂/∂t to its wavefunction. Based on this wave arising from the circular motion of FCs, a particle's energy must propagate at the speed of light. This requirement extends beyond the constancy of the speed of light in all inertial frames proposed by special relativity. In the rest frame of a particle, precisely half of its energy moves forward and the other half moves backward at the speed of light, creating an apparent zero velocity. This constant energy propagation speed implies a constant energy flow rate, which then yields the Schwarzschild metric of general relativity. This work demonstrates that the principles of quantum mechanics and relativity are inherently linked to fundamental particle spins. Furthermore, the spin theory enables the derivation of electroweak coupling constants from first principles. The theoretical fine-structure constant derived from this spin model (without experimental input) exhibits strong agreement with experimental values (a discrepancy of only 0.14%), and the predicted electron charge (1.603×10^-19 C) closely matches the experimentally determined value (1.602×10^-19 C).