I've got a lot to say to you, Titan, but I think most of it is in here, and I'm urging you to read it with some attention, even though it's long. https://clusterbusters.org/forums/topic/6213-basic-non-busting-information/ One important thing you will see in there is that you can open up your Imitrex injector and get 3 shots instead of one. Here's a direct link to that info. https://clusterbusters.org/forums/topic/2446-extending-imitrex/ Are you in the US? Because here, Imitrex is very expensive and most prescriptions are very limited, and it's hard to see how you'd be using so much. Yes, the theory is that Imitrex is for attacks that are not handled by oxygen -- taking too much of a triptan (Imitrex is sumatriptan) is bad for you and can not only extend your cycle but cause severe rebound attacks (triptan overuse attacks). Given my misgivings about your doctor, I have some fear that your oxygen setup might not be correct. You should have a tank or cylinder, a regulator, and a non-rebreather mask. (Much more detail in the post I linked you to.)
I also want to note, not as a putdown but just for clarification, that there really is not such a thing as a "cluster migraine." Two different things, cluster headaches (CH) and migraines (although some medications, such as Imitrex, work for both). A small number of people have both, and for an even smaller number, a CH attack is always preceded by a migraine attack. If a doctor in the US said you have "cluster migraine," I would be very wary of that doctor. It seems more commonly used in other places, such as the UK (but it's still wrong).