I can empathise Massively. Although I havent lost my parents yet, I know I will one day.
Howver I have had to deal with watching both of them become broken people in the last year (In particular my Dad) you may know I lost my brother to Suicide last year, although it was very bad for me the long term helping my Dad Grieve has been almost more difficult.
Steve I know your a sensible bloke, I would seriously advise some specialist councilling even at this stage, having a third party too talk too is exceptionally important and if they are a good councilor they will give you strategies on how to cope with your dad....
TBH sometimes I grive for my parents too as they are not the people they once where, bear in mind yourself and dont supress too much.
I have had a bit of counciling, not so much for my own well being but on how I deal with my dads grief and too help him.
If you havent got someone impartial too talk too I would say the above advise essential..
Ever you want a chat even over PM etc just send me a message, my thoughts are with you and you lads..