...or, how do you know G-d speaks through your conscience?
Consider this. You want to feed your dog a little later figuring that the bowl was half full this morning before you went to work and he's already eaten his fill during the day, so he can wait till before you go to bed. You're tired and want to sit down.
Your conscience should stop you. G-d doesn't agree with us all the time. Sometimes yes. Like when you feel you should intervene and stop a mugging. Sometimes not. Like when you try to rationalize having the last piece of desert in the refrigerator. You know you should let someone else have a go at it. It's better to be selfless than self-centered. You know G-d is talking when you hear your own voice somewhere deep inside say, "but your wife might like that after lunch tomorrow. Let her have it. It'll put a smile on her face and you'll feel better about yourself having done a good thing for her."
So, you say back, "yes, I should see to his food and water now while I am up and not later. It's not fair to him. He can't get his own dinner. I'm responsible for him."
In general, G-d will agree when you're doing the right thing, but not necessarily without criticism. He'll point out little points that might help for the future. Or simply a little later. He'll console you when you're being too hard on yourself, and nag you when you're not being hard enough, when you know that you can do better and the easier it would have been, the more He nags.
However, G-d doesn't ever get nasty. He loves you and is always trying to improve you. The best you that you can be is in His interests for the world, this we know and take on faith. He doesn't set silly standards though and understands you need help along the road.
It's in your conscience where you know. The kind of knowing that doesn't need proof, it just is. The knowing that has an honest balanced and fair view of you. Neither sanctimoniously flagellating nor overly excusing egomaniacal. You know. So don't be afraid to accept it. That fear is what costs us. What holds us down.
I've been there. Where faith is so hard to have. Where your only option seems to be that which the cold uncaring part of the world says is exclusive: give up. I didn't give up. I don't say this to crow. I say this to let you the reader know that the darkness in my life at that time passed, and my marriage is here, my wife is here, my life is here. It would not be had I given up. The bad times don't last forever and you know that.
So stop running from what you know.