My understanding of this is that there is no limit on the amount of memories that the brain can store. I think this is still debatable, but even if there is a 'memory capacity', then it is so big that it can be considered 'infinite'.
For the billions of brain cells which interconnect, there are over ten million billion junctions. The memories are dispersed over these webs.
Some experts comapre memory to a hologram. If you smash a hologram, each broken piece will still contain the complete picture, only fainter.
We never forget things, we just have trouble recalling them.