Filtering a signal only around 0

I'm new to signal filtering and I'm not sure how to perform the following.

I have a signal that I want to remove noise from, as seen by the unfiltered signal here

The filter used to produce the orange signal is a butterworth filter. What I really want to do is only filter out high frequencies occuring around the y = 0 point (the inflection in the wave) in order to smooth it while not interfering with the morphology of the rest of the signal.

Your help is appreciated!

[dpb]

You can try multiplying by a gain factor near unity at whatever you think is "near" 0 that scales rapidly to small values for range of input signal, filter, then inverse gain the result. That should leave more or less continuous.

Or, you can just try it and see if can live with discontinutiy by just selecting y(y<thresh) and operating on it.

I don't know of any specific work of the type but there well may be if look for it in literature.