The second half of your sentence does offer a different point of view from the first but that's why you have a contrasting conjunction ("but") instead of a joining one (such as "and").
Note also that grammar doesn't necessarily cover the logical coherency of a sentence. This sentence makes no sense by itself but is grammatically correct:
I hate avocados so I eat them everyday.
If you want to split your sentence into two, you can do that:
I used to hate planning and I still do. Once I'm done planning, everything's easier.
These sentences are both grammatically correct but because they seem to be somewhat at odds with each other, they sound a little off. You can fix that by adding a word to connect the two while acknowledging the contrast:
I used to hate planning and I still do. Once I'm done planning, however, everything's easier.
I used to hate planning and I still do. However, once I'm done planning, everything's easier.
Here the word however fills the same role as the word but in your original sentence.