Four-part harmonisation.

Hi folks.

I'm working on SATB harmonisation through Challan harmony. I have almost no problem about 'rules' (i don't like calling them as rules, but you get it). Also, when I feel the melody I write it instantly and other parts are written easy as melody. But at some points, I get stuck so hard, I almost wait in front of the page for 20 minutes or more.

First of all, here is what I usually do briefly:

Ring the given part.

Imagine the cadences and modulations.

Write the soprano if bass given (and vice versa) until the first phrase ends.

Harmonise the A and T to the end of first phrase.

Now, if my soprano (or bass) is satisfactory, i do the rest easily. But sometimes, this is the part I'm suffering, I say 'yeah my soprano seems satisfactory', and I do the harmonisation but just before the end of phrase, or two measures before, I find myself in a situation in which I have to make an harmonisation error in order to continue. Then I check what I did in the previous measures, and they look great so I don't want to change the melody, but there is got to be a mistake back there (in spite of parts written without an error). Sometimes the error seems incredibly inevitable, it forces me to erase all the measures i wrote. And you know how frustrating that is...

At this point, I think new points of view might be very useful for me. I'll be thankful if you share your experiences.

Cheers.