How to handle this “open ended” penance?

I went to confession at the beginning of December to confess something I’ve been struggling with for quite a while (lust…you know the deal…) and the priest told me my penance was to completely fast from the internet until Christmas, which I did. He told me that once this was completed, to continue the fast if I feel tempted to give in to it again. He did not specify a time limit. So…literally yesterday I gave into it again. The thing is, the reason he gave me this penance was because the internet “puts bad images in my mind and tempts me” however I actually find that my temptation lies moreso in my own vivid imagination rather than going online. My question is, do I continue with this penance/advice even though I feel it does not help me, just because he told me to? I’m prone to scruples and I’m wondering if I’m disobeying God by not listening to the priest.