The previous advice is good, but I'd like to add my 2 cents.
When someone comes to me and says they hit "X" club poorly, but many others are fine, my first thought is that it is a mental issue.
You may have hit it poorly in the beginning and now there is some anxiety with the club....
You may be subconsciously attempting to hit the ball harder because it is a longer club....
You may be subconsciously trying to help the ball into the air due to the loft....
All of those are mental issues. The five iron is the first club that really steps into the "long iron" category and is going to show any issues very readily. One thing I would ask you is how do you hit your 4 and 3 irons? If those are not used at all or hit just as poorly then here is what I would recommend:
Start with a pitching wedge and hit some half swing shots. Just get loose and focus on making solid contact. Slowly work your way up to full wedge shots. After you feel loose and comfortable, bring out your 5 iron. Start by hitting half swings. Use the same general feel you had with the wedge. Don't focus on distance, just focus on solid contact. As you get more comfortable, lengthen the swing. If you ever revert back to hitting it very poorly, come back to the half swing, or even back to the pitching wedge if necessary. What this will do is build your confidence and get you used to hitting that club the same way as the shorter ones. It is important that you don't even worry about distance when doing this drill. I commonly pick a flag that is very close to me for a target on purpose (i.e. 80 yds away, but hitting 7 irons). I can still evaluate the accuracy without the extra mental issues of distance. Hope that helps.