I was going through "None but the Hungry Heart" devotionals, and when I saw your question, I saw this in there:
“There are believers who need the reminder that deeper than mind and feeling and will, deeper than the soul, where these have their seat, in the depths of the renewed spirit, there comes, at re-birth, the Holy Spirit to dwell forever. His indwelling is there, first of all, and all through, to be recognized by faith. Even when I cannot see the least evidence of His working, I am quietly and reverently to believe that He dwells in me. In that faith I am restfully and trustfully to count upon His working, and to wait for it. In that faith I must very distinctly deny my own wisdom and strength, and in childlike self-abnegation depend upon Him to work."
I'm one of those believers who absolutely needs that reminder. And every feeling or thought that makes us think "I've fallen from grace", is supposed to make us WANT Christ and God's help more, instead of thinking that we're all alone, there is nothing left and that we should perform some magic rituals in order to get back somehow. I must believe that the old self is dead. Even if I'm still trying to do something (or everything) with my old self, with all its unbelief, with its hard-hearted and vain desperate efforts that don't lead anywhere.
If I give in to this thought: "God has left me, He has abandoned me, there is nothing left", and I have, it's unbearable. How could I go on? With each struggle, I've learned to just go to Christ, trust God to be good and gracious and absolute love, without believing any of my feelings that might tell me otherwise. I didn't perform any tricks to get His Spirit in the first place. He did it. I didn't earn it, did I? I don't get any of the glory. He doesn't give us scorpions when we ask for something good, and He'll be running towards us with open arms to meet His prodigal son. He'll find the lost sheep. He wants us to go to Him, He wants to heal us, He wants it. He wants the sinners, the broken-hearted, the weak-willed and those of little or no faith. He wants us so He can help us, mold us, and work through us.
If you think you're fallen from grace, if you think the Spirit has left you because of something you did or didn't do, then where does this yearning come from? This question "how to get back to Christ in fellowship", don't you think it's the Spirit working in you, drawing you closer to Christ, empowering you to approach Him as you are, removing every obstacle your mind can put in between you and Christ?