Not Trusting Myself To Love God Until The End...

I can't tell if the way I am feeling is a good thing or a bad thing... I keep having these fearful thoughts that I will let the ''fire'' of God's love in my heart burn out 1,2,5,10,20, or 50 years from now... Jesus said only those who abide in me UNTIL THE END will be saved. I am so worried that I will not be strong enough to love God that much until the day I die... there are times where I weep over Gods love and His beauty but then there are times where I am ''hard'' and really don't care about God's love and that scares me... is this what David was feeling in the psalms when he said ''create in me a clean heart, renew a steadfast spirit in me'' and ''my sins are piled up so high I can't see''? Anyone ever felt like this?