I use to suck at focusing. I would zone out in classes, be unable to get what I needed to done at home. This lead to trouble with grades in school even having to retake a couple of high school classes. As I started wanting to pursue creative endeavors I found myself unable to get started or if I did get started I would stall out on the first difficult step. This bothered me of course but instead of facing it right away I would distract myself with TV and video games (I still enjoy both in moderation). This was all to help hide the pain of not working towards my potential.
After all of that difficulty I eventually decided that I need to just keep going. Going after it gets hard, after I do something wrong and even after I no longer feel like doing it. Assuming it is working towards a goal or virtue of yours otherwise you should totally stop. This is not to say work until you burn out but to say work a little longer than you did yesterday. And be okay with the uncertainty of the difficult problems you run into. These are meant to be teachers and if you do not yet know the answer than there is just more to learn.
Sitting with difficult problems day after day making what progress you can will build focus. It will take time and is something that must be practiced regularly or will slowly decay. As you dig deeper and deeper into a problem you find that you can start to enjoy the work instead of feeling inconvenienced by it.
I use to feel unlucky and that my short attempts to build things were all I had in me. I use to think that everyone else must have been born with some innate ability to focus that I was just not given. I use to look in wonder at the projects that people built wishing that I could do something like that. Now I have realized that everyone else has to build their focus everyday. And that other peoples projects are not single acts of sheer focus but small consistent steps forward focused on progress each day.
Our attention and what we focus on are our most valuable resources. Building control over focus will change your life. This is a very gradual process so you must be patient. Remember this will just slowly become your new norm. Keep a lookout for you starting to do all of the things that you had only dreamed of doing before.