I simply say that I am sorry he feels that way; that I wish it were different and that he believed my words instead of his own fears.
But I also let him know that me being sympathetic to his fears does NOT mean that I will be accepting that line as an excuse. It just means that I wish he felt differently. Nothing less, nothing more.
I think my H was most surprised when I started saying to him, "I don't accept that as an answer." He was so used to me giving in to his excuses that when I started saying I would no longer accept that as an excuse he would stammer at me, 'Wwwhat? What do you mean?' I would reply, "I'm sorry you feel that way but it is not an excuse to not do anything and I am still expecting that things go differently next time." All of this was said in a compassionate tone and I really WAS sympathetic to his problems and hangups. But I just stopped accepting certain excuses from him and he stopped giving them.
Just because you're sympathetic does not mean that you have to become paralyzed with inaction and wait for him to overcome his problems. Kwim?