Is it ok to call myself stupid in an email to a professor explaining why I have not completed my project yet? Thing is, dude was happy with my work in a research course with him over the summer. I had a small portion left to complete the project which I offered to volunteer to do. He immediately ordered the supplies I needed to conduct this last portion of research, and that was 2 months ago :(. I sort of never considered until now that I should have finished it in 1.5 months maximum regardless of the fact that I was volunteering on my own time. In my defence he's been on pat leave (although he attended informal weekly departmental meetings on campus). 

How do I explain this to him in a way that shows I acknowledge my mistake and that I am still interested? Do I stand a chance of mending the relationship? I intend to finish the work over the next 2 weeks.
I would pose two questions:


Is the professor unhappy with you? 
Did you really do anything wrong? 


If the professor has not said they are unhappy with you, then I see no reason to give an explanation; keep it up-beat.


  Dear Professor, Just a status update on the X project. I've finished X, has taken a little longer than I anticipated, but I think I'll have it wrapped up in the next two weeks. Looking forward to it!


If the professor is unhappy, then the question becomes whether you did anything wrong. If there was no deadline and you're not getting paid, then it's a bit rich of the professor to be angry. You can push back as much as you're comfortable with; here's a reasonably polite version:


  Dear Professor, Sorry for the slow progress on the X project. Given that we had never discussed a deadline (and that I am not getting paid), I hadn't realize you had expected this to be finished by now. Still, I do want to finish the project, and will pick up the pace. I think I can be finished in two weeks; would that be OK? I'll keep you informed on the progress. Thanks,


If the professor is unhappy and you did do something wrong, then a quick apology is in order. 


  Dear Professor, Please accept my apology about the slow progress on the X project. I am still interested and definitely want to finish it; I had just [stupidly] been doing the work pretty slowly and hadn't managed to keep the schedule we agreed upon. I think I can finish the project in the next two weeks; would that be OK? Happy to discuss in person if you like, just let me know. 


As to whether to include the word "stupidly", I think it depends on your personality a bit -- as stated in the comments, you shouldn't seriously suggest that you are stupid, but I think it's fine as a way to be a bit self-effacing.

(Note, my answer is from a Western/US perspective)