What to do with an unfinished research project you previously worked on? The situation is as follows: I am a third year PhD student in theoretical biophysics and recently had my first jointly-authored journal paper published. My advisor thinks that I am on track, though to be fair a lot of the credit goes to another student who did a substantial part of the work for the paper. In any case, before working on this paper I was working on a completely different project, which my advisor asked me to abandon in order to work on this paper. We never talked about the previous project again. 

Now I am wondering what I should do with the previous project. I have quite a fair amount of results and put several months of work into it. However, it is hard for me to put the results into context and even convince myself that they are interesting enough for a publication. My advisor is not an expert on this particular topic, and I think that to convince him to revive the project I would have to come up with a brilliant idea of what to do next (which I also don't know at the moment). 

So I am torn about whether I should put more effort into trying to revive an old project my advisor told me to stop working on or to give up the project altogether. The last option would be a difficult decision, because of all the work I put into it, but if I can do better research instead and it does not hurt my chances of finishing my PhD on time, I would be able to accept it.
I think this question is difficult to answer without knowing the details of the project, and it's really something you should work out with your advisor. But let me offer two perspectives here that maybe helpful. First, you said that you have spent several months on the project. On the grand scheme of things, several months is nothing. A successful project from start to finish can take 1-2 years (and oftentimes even more). So if I were you I would not fret too much about the time lost to the project, should you choose to simply move on.

Secondly, have you heard of the concept of sunk cost? It's basically an economic concept referring to effort/time/capital spent that cannot be recovered. And the key here is that sunk cost should not factor into future decisions. What is lost is lost, and the irrational desire to recover what is unrecoverable usually leads to more lost time and effort. So you should try to evaluate the merit of the project independent of how much time you've spent. If you find it hard to sell the project even to yourself, as you wrote, then maybe it's time to move on.