Obviously no one here is a Time Lord, so no one can know for certain what he must be going through, but a little speculation never hurt.
This has always been a thought that has gone through my head a lot since I became a fan of the show. In fact it's probably more interesting to me than the silly notion of "Doctor Who?" that is apparently the season arc this year.
Now we've seen in the past that the Doctor retains all the memories from his previous incarnation, yet Ten treats his impending "death" as the end, until of course he regenerates and it's all fun and games again.
Obviously a regeneration is more than just a facelift, but how much more? Physically he's in the same body, but each new Doctor is treated as an entirely new person. And each apparently goes through that "foggy" period where they have trouble remembering things (lasted especially long for 8, cause of the surgery).
Just something I've been mulling over lately.