He said: "Newcastle deserved to win, but the result is heavy for us because yes, Newcastle played better than us with more intensity, with focus, but in the second half, we had some situations, some possibilities to score and draw level, and maybe the match could have changed.
"But this is our job, it's my job. I know football. Maybe after three months of hard work to achieve where we are in the table, it's possible to have a bad day, and today for us was not a good day."
Guidolin, however, insisted his players had not taken their foot off the pedal after reaching 40 points.
He said: "No, I think no. We need some points for mathematical security to stay in the Premier League. It's not done yet, so we need some points. I don't know how many - one, two three - but it's important not to relax.
"Football is particular and I have seen many surprises in my career, so it's important to be focussed and be aware."