Yep, Lorric is correct. A decent playtester who'll give you rapid, detailed and regular feedback is more valuable than any other aspect of FREDding when it comes to helping to keep you motivated.
One other suggestion - and I want to emphasize early on that this is one to be used sparingly - is that if you're stuck on a particular mission, consider adding a new ship (assuming your setting allows it). I find that including a new ship that I haven't played with before can add new tactical dimensions to a mission, and make it at least a little fun to play on your thirtieth run through, because you're still probing and investigating this new addition to the fleet that you probably haven't had time to learn about in the same way we have the existing FS fleets.
But again - you've got to be careful with that. Add too many ships and feature creep becomes an issue. Get too dependent on new ships to drive storylines, and you have a whole new set of issues - one of the things I found with Shadow Genesis, for example, was that it dumped a whole bunch of new ships on me over the course of several new missions, and I never got to find out enough about them to learn how they worked and how I should deal with them, or even which one was which. Worse yet, if you do this too much, you soon find yourself wanting specific ships that you can't get from the wiki, and that slows FREDding [i[way[/i] down.
So yeah. Use it sparingly. But consider it if you're really struggling every now and again.