Fiddlesticks may not be the kind of champion you may think it is. His playstyle revolves around getting your teammates fed and engaging in teamfights. Every time you gank, you do it to give your teammates a kill, not an assist. If you take the kill a few times, it's fine, as long as your team is fed.
You never wanna be the only one fed on your team, since it ends with loss every time you do. Fiddle's lategame sucks and considering the enemy will focus you and your team can't do anything about it, it only makes sense that you lose.
Trust me, if you get your teammates fed, you'll eventually get fed yourself too. You could pick up some free kills with your ult, once your whole team is fed. That's how Fiddle carrys.

Pros:

Good Engage With Ult
High Amount Of CC
High Damage
Good Vision
Quick Jungle Clear

Cons:

Very Squishy
Bad Mobility
Vulnerable To CC
Bad Against Tanks
Basically Useless When Behind

EARLY GAME:

Your main priority is to give your teammates the kill every time you gank. When you ult and see the enemy is low, stay away from them and let your teammate get the kill, however if they're too far you can take it. If you W a low HP enemy and wait for that satisfying final tick of the W to finish them off, don't do it if your teammate's about to get the kill, no matter how good the succ feels. If they're not you can take it in that situation as well.
Try to stay away from the enemy Jungler, since fighting them doesn't give you much, unless your team is about to help.
You can gank early before your ult, since most laners don't expect Fiddle to gank without his ult. Make sure to focus the ADC when ganking Bot, if they haven't ran away yet. Put down a Control Ward in the bush as you approach it, so that they don't back off. Only gank when the enemy is pushing if you don't have ult and fear the ADC with your Q and start using your abilities on them.
Again, once they're low, let your teammate get the kill, while running above the enemy so they don't escape. If your teammate can't catch up, take the kill.

MID GAME:

At this point, you and your teammates should be fed enough to win fights. If you're not then at least make sure your team has a little bit of kills without dying too much. Remember, the reason you don't want to be the only one fed is because even a fed Fiddle is easy to beat in Late. You can take Drakes after ganking, or seeing the enemy Jungler is at the other side of the map. Make sure to put your effigy or a control ward in the pit, however. A little tip on how to ensure you get the Drake, if you feel like someone's about to steal is to use the last tick of your W to get the Drake low enough for your Smite to kill it. Kind of like Nunu & Willump's or Kha'Zix's Qs.
Try using your R as soon as it's up and when you do, make sure to do it over a wall. Put some vision at that wall too, in case the enemy was ready for your R.
Make sure to be in every teamfight no matter where you are. In fact, if you're too far away from it and you feel like your team may not win the fight, tell them to back off immediately. Then rush over there and if your team stalled for you and you have R, use it to surprise the enemy and win the fight. That's way too situational, I know.

LATE GAME:

So you're probably wondering "You told us Fiddle is easy to kill in Late, so what can we do now?" Well, I got a simple answer, and that is Hextech Rocketbelt! Of course, you don't always have to buy it, but most of the time, it's pretty effective. Basically it removes most of Fiddle's flaws! With Protobelt, you now have both mobility AND good sustain! Now this doesn't mean with Protobelt you can carry with Fiddle even in late if you're the only one fed, but it does mean that you won't die right away in teamfights. In fact, you now have a lot of damage, sustain and mobility! You're perfect at this point! Make sure to always engage with your ult and MAKE the enemy team focus you, while your team beats on them! And don't worry, when they do focus you just press Zhonya's Hourglass and the game is won! Protect your team at all costs and delete the enemy squshies. That is, in my opinion, the best way to carry with Fiddlesticks!

Now I will be updating this guide sometimes, especially for Preseason which will be a lot! Though most of it is pretty much the same. Fiddle isn't any more or less of a champion in Preseason.