This paper addresses the self-interference (SI) cancellation in a full-duplex radio transceiver. In particular, we focus on shared-antenna based full-duplex transceivers where the self-interference coupling channel is always frequency-selective and can also be strongly time-varying depending on the antenna matching characteristics and reflections from the surroundings. A novel digitally-controlled RF self-interference canceller structure is described, being able to process the signals in a frequency-selective manner as well as track adaptively the time-varying SI features, stemming from the fast digital control loop. A complete demonstrator board is developed, reported and measured, incorporating both the RF processing and the digital control processing. Comprehensive RF measurements are then also carried out and reported at 2.4GHz ISM band, evidencing more than 40dBs of active RF cancellation gain up to 80MHz instantaneous waveform bandwidths. Furthermore, real-time self-adaptive tracking features are successfully demonstrated.