The problem is that your forms directory isn't in your include path. Two possible solutions would be to:
1. Add the forms directory to the include in the contollers init() function. The bad is that you have to do this for every controller, resulting in lots of repeated code.
2. Make a plugin to add the forms directory. This is probably the better solution. You can find an example with model loading
here. Some minor alterations will get you up and going.