Should I withdraw my paper if I find another paper published with the same idea?
It's a slightly grey area but assuming the journal paper in question was not publicly available before the submission date of your conference, I think you are fully entitled to publish the paper if accepted. It's a parallel result, which means there was (verifiably) no wrong-doing on your part. If accepted, in the "camera-ready" submission, you can add reference to the journal paper with a note that their results were developed in parallel (with mentions of the dates involved in a footnote, for example). You get your publication, the authors of the work get their citation, and the conference themselves know that there was no wrong-doing involved.

So again, you don't have to withdraw, but you can if you want (e.g., if you don't want to be associated with a weaker form of the result, or if you are a really nice person and feel that the slot at the conference might be better used elsewhere).