To Accept or not to Accept

I've recently taken on another commission and whenever I'm asked I always instinctively say yes, provided it is not something outside my normal genre of subject matter. Currently, I don't put a premium price on these works but that is about to change. I think a 10% charge is fair. The dilemma I am starting to face is, should I in fact accept them at all? The reason I say this is because it interrupts my work flow when I'm trying to get a body of paintings together. Although I take great pleasure in seeing people happy with my work and it having meaning something important to them, the benefits as I approach my mid-career are becoming less. Artists need to explore ideas and it is often detrimental to the forward development of their careers not to play these out. I have to work full-time which brings me to the second part of my post. I've just heard that Artbank will not be paying me for the work they've purchased until the end of the year, because they will not be able to get their hands on the work until then. I can understand this, but when they agreed to buy the work they knew this was the case but promised to pay me in March, so now I've got to wait until at least Christmas for payment. In a round about way, I'm explaining both why I need commissions and also why I work full-time because regular money into the artists’ pocket is almost impossible unless you are in blue-chip land. Whilst I sell a reasonable amount of works each year and for a reasonable price, I find it increasingly frustrating to obtain funds for what you are owed. Also, this is why I get frustrated with commissions because stopping to do them, stops me painting new works which I need to do to move my art career to the next stage...