Maybe one of the things you installed has a memory leak. If a program allocates memory for a resource and doesn't release it properly, it will gradually use more and more memory. You can run top from a command prompt to see what is running and how much memory it is consuming.
You can quit top with the q button.
You can tweak raspbian yourself and disable all unused services. The startup text gives a good indication of all what is started at bootup. It's just not always easy to figure out where and how a service is started. Also, some services depend upon others.
Another thing that can be tweaked is the kernel. You will need to rebuild it with some of it's drivers and functions disabled.
Another approach is using something like yocto to build a minimalistic linux. This basically works opposite. You start with nothing and adds the stuff you need.
You could also buy the pi2 with double the memory.