Does this sentence make sense?

Pan fyddaf yn clywed pobl yn siarad Cymraeg , mae'n ymddangos mor hawdd , fel anadlu , ond yn yr achos hwn mae gen i asthma.

If someone could tell me if this sentence makes sense, that would be lovely!

It does make sense, but instead of "mae gen i asthma", it would be better to say "mae asthma arno", this is the usual way of talking about ailments and suchlike. Also, I think "yn yr achos hwn" sounds like a bit of an anglicism; you could say "os felly" instead.