John 10:28-30 should be of tremendous comfort
28 And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
29 My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand.
30 I and my Father are one.
When you accepted Christ, you made an irreversible decision, and Christ will never let you go.
When I first got saved, I struggled a lot with the issue of wondering how I could have been so joyful to have been saved, yet still have thoughts that were so hostile to Christ.
And I'm not exaggerating when I say that some of those thoughts were worse than anything I had ever felt previously. Part of this was due to my nature as I am a bit obsessive at times (sometimes it's a good character trait, and it's a good thing when it comes to Christ). I also believe that some of this was due to outside influence. Satan can and will tempt us, and part of that could also be in the form of trying to convince us to change our position on Christ.
As my pastor put it once, if you're worried you may have committed the unpardonable sin, then you really couldn't have done it. God doesn't leave you once you're saved, and if you're concerned about it, it shows that you're sensitive to trying to please him - not something a blasphemer would do.