The most direct way this changes is usually by some type of intentional "approach" (ie not avoidance) of the hardest to tolerate memories and feelings - which seems to let your brain realize they are actually tolerable (now, in a safe environment, etc - there may well have been a reason for avoiding them before). When people do this (often with a therapist) numbing tends to go down.

Not the only option or anything! But often the most direct one.

Ah, yeah, that can be a challenge. It's something that comes up for sure, but it's hard to give a general answer to.  If there are memories that must people would consider likely traumatic but they feel kind of numbed out too that's often something a good therapist can coach you through - but it's pretty individual.  

Another option to consider is looking through some of the DBT handouts on recognizing and naming emotions, which have very concrete descriptions that include physical sensations etc - sometimes this can help if part is what's happening is you're feeling very cut off from emotions.

This sounds like what is often part of emotional numbing in ptsd. People often think about it as part of a way to stay functioning and not be overwhelmed by negative feelings - but at the cost of feeling distanced from positive feelings, too. Does this match what you're experiencing?