Wow, Celtic. Amazing questions. And probably as many answers as there are readers here. IMO, I think there are lots of things that hinder our ability to hear God...all of what you've suggested...religious background, self-esteem, fear (on so many levels, including fear of responsibility - fear of being wrong and unintentionally hurting others with what we think we've heard)...for me, there are very clear moments when there's no doubt that it's God, there are others when it's very clear that it's my own ego or self-destructive tendencies that are speaking...and then there are the in-between moments when it's not clear who is speaking. What I know for me is that I've given God permission to do whatever it takes, whatever He wants to get me to listen, and I believe that He prefers to use other people to get His messages across to me, perhaps because He knows that that's my greatest fear and weakness, trusting other people. Again, for me and my journey, it's all about love, learning to love others and learning to allow others to love me...so it makes sense that He prefers to use people to get that particular message through to my mistrusting little brain! LOL.

One other way that He has used to really break through with vital message is through dreams. Dreams have often brought the answer I've searched for, or direction, or comfort or clear glimpses of Him and His yearning for relationship. Those are precious treasures that remain as vivid today as they were when I first dreamed them.

I still believe that God will use whatever, whomever He knows will best get through to us. I know it's Him when whatever/whomever it is breaks through my darkness and pain and brings light, comfort and evolution.
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When you don't like a thing, change it.
If you can't change it, change the way you think about it.

(Maya Angelou)