A close friend of mine (who was a Protestant – and I was too at the time) taught me this:
With God, trying equals success. We must try. If we try then that is what counts.
I don’t know if that is true or not. But it seems to me that what we can do is try. We don’t always succeed. But there is such a thing as “not trying anymore”. So always try. Don’t quit because you forgot a day. Keep trying. Maybe make the novena longer (add a day or two or more) if you want to and are worried about having missed a day.
I once began a 9-month novena (the prayer Fulton Sheen wrote to pray to adopt an unborn child and protect it from abortion) – I didn’t know anything or expect anything except that I wanted to pray for the unknown unborn child. I was unable to pray every single day. But I prayed when I remembered to pray (a helpful card at my desk somewhat hidden from obvious view of others but visible to myself). I think it worked. And when the unmarried pregnant co-worker very nearby (I didn’t know she was going to get pregnant only a few months after I started praying) was sure she wanted her baby I kind of dropped off the praying every day. She married her boyfriend.
Anyway, I haven’t prayed too many novenas at all. But they all seem to have had some results so really I should try praying novenas more often.
Prayer works. Don’t neglect it.