The 155 would be easier for you in terms of turn initiation and overall manueverability, but in all honesty, the 158 is not that bad for free riding and the longer a board is, the better it handles speed. Since you already have it, I would advise you to just hang on to it and work on improving technique.
To that end, going as infrequently as you do, you will have a slower learning curve so take that into consideration when comparing yourself to other riders. The main thing to focus on at this point is your foundational riding skills and concentrate on good, efficient body movements to achieve the kind of board performance you are wanting. To this end, I would encourage you to peruse this section and read through the many posts regarding basic riding. In addition, read through our "how to" section as well as the video lessons that I have produced as well as Snowpro Rick`s. When you do get to higher levels of riding, Snowboard Addiction has top notch material but to be honest, you are not at that point yet.
Getting more comfortable with speed comes from confidence and confidence is gained by time on the board. Really focus on good technique. As a general "tip" really concentrate on using the board`s twist to initiate turns and use of the front foot to accomplish this through pressuring the edge of the board. The best riding is done from the feet up so really get some flex in the ankles and knees.
As you get faster, resist the urge to lean back as this will simply make you go faster and your turn initiate and control slower. Ideally, remain centered between your bindings and when initiating turns, shift your weight onto your front foot a bit. The best is about a 60/40 weight distribution. It is also important not to become static and stay forward all the time. Remember to return back to center as your progress through your turn.
As for board chatter, your best option to deal with this is to work on riding at higher edge angles. This will naturally get you closer to true carving. Carving smooths out your ride because you are now cutting through the snow rather than bouncing along on top with a flat base....