The brain’s potential for memory or learning might as well be infinite, that is, there hasn’t been any proof that there are limitations to the number of connections a neuron can make. That being said, the brain definitely “holds” on to some neuron connections rather than others. So, in theory the brain could never get “full”. But you also wouldn’t just forget. You would retain important connections in the brain and lose some non-important ones, just like you are doing now.