Here are a couple of addons, both very similar, that can help you to analyse your wins/losses and let you to identify which setups you seem to struggle against.
This is a very detailed addon that stores a lot of information about every game you play. As you can see from the screenshot below, it’s packed full of features and lets you create very specific searches, even down to your performance on individual maps.
While this doesn’t produce the detailed reports that Arena Historian does, it’s pretty light-weight. It doesn’t record the specs used by your opponents, just the class. However it does have a couple of nice features, such as being able to easily produce a summary of the last games you played (“/arenalog history 2v2 20” for example).
Both tools are useful in their own way, you may even wish to run the two addons side-by-side. While you may initially think “oh, I know what I struggle against”, it can be quite surprising to find that certain setups are dominating you without you realising. Looking through the reports before making this post, I realised that we’ve gone 0-5 against Rogue/Shaman/Spriest without even realising it. I guess that’s one setup we really need to work on our tactics for 😉