You have to start, like with anything else, with very small steps from where you are. Have to learn to feel about things here and now, prefer one "safe" thing over another, get use to doing that and feeling the preference and then gradually work your way back.

Do you prefer steak or chicken? How do you like your eggs, etc. What time of day do you like? (my time is 4:30 p.m. :-) What day of the week is your "favorite." My T and I changed days my session was on and it made a difference. Looking at that (the why it made a difference) was good practice for seeing smaller differences and seeing how I felt about my T/therapy, etc. Eventually you'll start tripping over triggers and things that remind you of other things and small pieces of things will come back to you and you'll begin to feel more/"better" about more important things.