If the private bytes keep increasing, it means you have a memory leak. Try DebugDiag, it is from MS and free, also a very good tool to tracking memory leak on Windows.
Use this tool is simple, first you create a rule to monitor your process with DebugDiag collection, it will create memory dump according to your rule, you can create the memory dump manually. Then you can use DebugDiag Analysis to analysis the dump, please set the right symbol path before analysis.
This MSDN article Identify And Prevent Memory Leaks In Managed Code might help too. This article points our how to find out if the memory leak is a native one or managed one. If it is a purely .NET manage leak, you can also use CLR profiler to debug the problem.