I have been working as an independent contracted employee for a start-up for 1.5 years, part-time (10-15 hours per week), and am the sole developer. I am also a full-time university student working another job 20 hours per week. In total, I'm a full-time student working ~30-35 hours per week.
My original responsibility for the contract employment was to build a web application. Before I started working, there was one developer working on a suite of Android/iOS apps (full-time) for approximately 5 years. Said developer stepped down right around the time I was hired, so progress on those applications stopped abruptly for a year.
In the last couple of months, I have been asked to take on some of the responsibility for (only) the Android apps. This includes bug fixes, content updates, etc. In taking on those responsibilities (because I wanted to help out, not because I was forced to or be fired), I asked for a pay raise because that work was outside my contract, and I had also fixed a major bug that was plaguing our ratings. I got the raise.
Now, my boss has put my original web application project on the back-burner (completely stopped development). For the next two months I am charged with implementing an entirely new flavour (a new module of the base app, essentially) of the Android app. This involves a lot of time learning how the previous flavours were built, fixing the inevitable bugs, etc.
Implementing this new app flavour is outside the scope of my original contract. Even though it's something I'd like to see come to fruition, it's really just too much for a junior developer (working alone) to do at 10-15 hours per week. I have to learn a lot of the massive code base and try to get something entirely new to integrate without a hitch, while having minimal advice from the developer who wrote the app. Moreover, I'm already stretched really thin with my other job and school.
I'm completely fine with doing minor bug fixes for the apps, but trying to integrate something completely new in what boils down to two 40-hour work weeks is stressing me out. I've worked on the problem for about 10 hours and gotten essentially nowhere.
What can I do?
I really want to tell my boss that taking on this task is just too much for me to handle right now. I want to stick with what I can reasonably make progress on without feeling stagnant and failing to deliver something worth releasing in the next two months.
However, I don't want to make him feel like I've wasted his time by saying "yes, I think I can do this" and two weeks later saying "no, I definitely cannot do this". I feel like I'm letting him down if I don't try to tackle this project. On top of that, I asked for a raise and he gave me it, because I said "I am being given more responsibility with this Android app, so I think I deserve a raise."
How can I diplomatically tell him that the responsibility is just too much for me to handle without causing him strife and feeling like he gave me a raise for nothing?