I can relay to this. I'm like you, starting things but can't persist. I grow tired and frustrated easily from mundane works such as make a pretty landing page, beef up our application's UI etc. Anything that requires 40+ hours of working is like a sea of burden to me.
However I had the great opportunity of having an awesome co-founder. He seems to be picking up where I left and finishes them off beautifully. Server side coding? check. Server side UI remake? check. Landing page details? check. I grew very interested in the fact that facing the same amount and type of work, why can he persist and why not me?
I think I had the answer this morning after quite of few months of introspection. The difference between me and him is that I look at how things were supposed to be, while he looks at how things could be. Let me make an example.
Remember the old story that half a glass of water can be both viewed as still a half remaining or a half missing, generating different moods? Well my tiny story is a twist to it. Say you were to fill a bigger glass with a tiny ink converter. Let's say it will take about 2hrs of mind-numbing work. After one hour with half a glass, I would look at the task and say, oh my, still a half to go. How grueling can this be? Then I'd feel frustrated and give up. Just like what I did to many of the tasks I started but never finished. My co-founder, I suspect, will look at the remaining work and say, oh my, only one more hour to go then I'll have this wonderful filled glass of water! That's why he stays up and working and never seems to have burn outs. I guess positive emotions never gives you burn outs. It is persisting under negative emotions gets you burn outs.
So I'm guessing the key is to catch yourself thinking in a bad way that is generating negative emotions, then turn the thinking in a better way that generates positive emotions.
I just realized this this morning. There should be a few points that I'm missing but I think I'll figure more out in the coming days/weeks. Meanwhile I am logging my thoughts and see if I can turn my emotions around and if that makes a (at least) personal difference. Hope it is useful for you.