I think the best way to do this would be to just think about why you think you lost interest? Was it lack of time, boredom, indifference to what you could achieve with the programme even although it might be good for you? Along with this, you could ask yourself why you wanted/needed the self improvement in the first place? And what motivated you to start? Asking these kind of questions can help to stimulate interest.
What I then tend to do is to pick a day and/or time for me to re-start my project but I try to view it as something new, rather than a re-hashed old one, this often helps with my motivation.
If you still feel lethargic then perhaps you need a different kind of self-improvement more suited to your personality and life-style. I have sometimes dumped a project only to become interested in something similar and enjoyed it more, simply because it was better presented or it seemed to suit me better as an individual. Hope this helps a bit.