food4thought,
I think, sometimes, we mis-attribute what "feeling the presence of the spirit" means. We very frequently participate in emotionally-driven situations and are told that those deeply emotional feelings are being "filled with the spirit", and they probably are; whether in part or in entirety. However I don't think that's the only way we "feel the spirit". There was an interesting article a while back, I'll see if I can find it; that suggested we Christians should be wary of evangelizing using emotional techniques (altar calls, passionate testimonies at big events, etc.) because, in their research, the people who 'convert' rarely ever become committed; they begin attributing Christian faith with a butterfly feeling and when they no longer feel the butterflies, they think they no longer feel God. It was a convicting read for me because I'm active with an organization called Emmaus that has youth retreats called Chrysalis; and while the retreats are focused on education and learning, there are moments that can't help but be emotional. And I have, myself, experienced young people who struggle when they don't get those deep emotional feelings all the time about God; and struggle to see and experience God in other ways, other than just the butterflies.
"How do we know" is a tough one. Paul suggests looking for fruit, but that can sometimes be tough; because we have a tendency as human beings to compare ourselves to others and never find fruit in ourselves. As cliche as it may sound; I think this one just takes a bit of faith. To know that the presence of the triune God, as manifested by the Holy Spirit; is something that exists in every corner of the world; including your corner.
I hope the others will chime in; though I'm not personally a believer that one can 'lose' the spirit, as if it's something that has to be 'earned' or 'gained'. While the choice to follow the spirits leading is left up to us; I don't think God ever stops leading.
