What do i do with my dog?

i have a seven mth old black labrador full pedigree.i had him six days when his heart started beating very fast,it was discovered he had a rare genetic heart condition,i contacted breeder she didnt want to know,he has really bad behaviour where he bites us and the children we have had behaviourist round a few times and he has said we are very unlucky as he is over boysterious to a normal pup.he has cost us a lot of money and we are insured people have told us to get him put to sleep but we cant do that and we cant rehome him cause nobody would want him,he is driving us mad and causing lots of stress in the home.he needs someone who has lots of time to train him and because he is so bad we cannot unfortunately.anyone any ideas.would be very grateful for your advise.we are at end of our tether.