You like your GP as much as I do my primary care doc--but I would not give him control over treatment of my cancer because, for all his good intentions, he doesn't know enough about that area of medicine.
What you have said about your GP's treatment says that he does not know how to treat Cluster. The approach for migraine and cluster can be quite different.
None of the meds you are using for are useful for Cluster. The Imitrex is the closest but the pill form doesn't work quickly enough to be helpfu. The injection is excellent but any doc will have to work with the insurance co. to get more approved (explore this question in the OUCH site, left buttons).
With two complex, if related, disorders, you really need a headache specialist.
To give you some background on Cluster treatments--to help see what you should be receiving--see the PDF file below and print out a full version of the abstract below.
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Cluster headache.
From: Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

or

(Orphanet Journal of Rare Diseases)
[Easy to read; one of the better overview articles I've seen. Suggest printing the full length article--link, line above--if you are serious about keeping a good medical library on the subject.]
Leroux E, Ducros A.
ABSTRACT: Cluster headache (CH) is a primary headache disease characterized by recurrent short-lasting attacks (15 to 180 minutes) of excruciating unilateral periorbital pain accompanied by ipsilateral autonomic signs (lacrimation, nasal congestion, ptosis, miosis, lid edema, redness of the eye). It affects young adults, predominantly males. Prevalence is estimated at 0.5-1.0/1,000. CH has a circannual and circadian periodicity, attacks being clustered (hence the name) in bouts that can occur during specific months of the year. ALCOHOL IS THE ONLY DIETARY TRIGGER OF CH, STRONG ODORS (MAINLY SOLVENTS AND CIGARETTE SMOKE) AND NAPPING MAY ALSO TRIGGER CH ATTACKS. During bouts, attacks may happen at precise hours, especially during the night. During the attacks, patients tend to be restless. CH may be episodic or chronic, depending on the presence of remission periods. CH IS ASSOCIATED WITH TRIGEMINOVASCULAR ACTIVATION AND NEUROENDOCRINE AND VEGETATIVE DISTURBANCES, HOWEVER, THE PRECISE CAUSATIVE MECHANISMS REMAIN UNKNOWN. Involvement of the hypothalamus (a structure regulating endocrine function and sleep-wake rhythms) has been confirmed, explaining, at least in part, the cyclic aspects of CH. The disease is familial in about 10% of cases. Genetic factors play a role in CH susceptibility, and a causative role has been suggested for the hypocretin receptor gene. Diagnosis is clinical. Differential diagnoses include other primary headache diseases such as migraine, paroxysmal hemicrania and SUNCT syndrome. At present, there is no curative treatment. There are efficient treatments to shorten the painful attacks (acute treatments) and to reduce the number of daily attacks (prophylactic treatments). Acute treatment is based on subcutaneous administration of sumatriptan and high-flow oxygen. Verapamil, lithium, methysergide, prednisone, greater occipital nerve blocks and topiramate may be used for prophylaxis. In refractory cases, deep-brain stimulation of the hypothalamus and greater occipital nerve stimulators have been tried in experimental settings.THE DISEASE COURSE OVER A LIFETIME IS UNPREDICTABLE. Some patients have only one period of attacks, while in others the disease evolves from episodic to chronic form.
PMID: 18651939 [PubMed]