I think it's probably easier, but you do have to try really hard not to get so wrapped up with the troubled child that you ignore the other ones. This is very easy to do.
Also bipolar and substance abuse are genetic. You can have more than one troubled child.
I think maybe its time for you to perhaps try therapy and maybe start getting closer to your husband, other family and friends. Church is one good way to meet people. There are many. Your son is not your only relationship. Do you have nieces and nephews? Yes, a child is irreplaceable, but that does not mean you can not have a full life without one.
God bless you. Take good care of yourself.