90% of the CPL is compass and stopwatch navigation. To be honest another hundred hours or whatever of P1 will make a pretty big difference to your flying (assuming you've just got your PPL) no matter what you choose to do during that time, but if you can concentrate on one thing make it navigating direct (not following line features, nor going via nearby big features etc.) to _small_ features, putting the map away most of the time (try to only get it out when you are either at a half-way point of a leg, or at the end of one). If you can get some confidence in the process of accurately holding a heading and making corrections at your mid-leg point, that will help a lot.
When I say small features, I mean stuff like a disused airfield, a distinctively shaped patch of forest, a tiny lake, a mast or similar. Try and make your legs 30 or 40nm long with a half-way or so check point, and structure the plan so that each leg has a decent half-way checkpoint so you can get an accurate fix mid-way and make a sensible correction to your heading.
Don't fix a nav error by going from where you end up to your intended half way point on a leg then continuing, rather, judge how far off track you are and in what direction, then use that to infer to what extent you mis-read the wind, and make a correction accordingly to route from where you are _direct_ to the end of the leg, using either double track error, or perhaps standard closing angle + single track error. Drawing 10 degree fan-lines either side of your track helps, but you can also learn to eyeball it, or use the 1 in 60 rule.
Learn to _really_ read a chart. Perhaps you could ask an instructor to talk you through the kind of VFR nav legs they did on their CPL. My experience was that even though I thought I could read a chart before the CPL, I actually couldn't, and my map reading was terrible - you need to learn how to look at small features, distinctively shaped patches of forest, town, coastline, tiny dots of blue that are basically a pond etc. to be able to get a really accurate fix off a chart visually.
If it helps, since you mentioned finding it difficult to manage flying, navigating and talking all at once, turn the damn radio off when you're en-route (in class G of course!), remember your collision avoidance is _entirely_ see and avoid, the radio cannot help you, and if you find it a distraction then don't talk to anyone...
Once you're confident with your nav as a result of the above, a few more transits on your own will help build confidence in your ability to put the nav together with talking on the radio.
If you haven't done any, and can afford to, I highly recommend doing some aerobatics. It'll do wonders for your general handling and confidence in the airplane and in your own ability to fly it.