I shall not discuss the question as to whether Buddhism is a pessimistic philosophy, but simply say that Theosophy is not Buddhism, neither is it any other particular religion, but the truth underlying all religions.
In answer to the question, "Is Theosophy a pessimistic philosophy," I say no.
Consulting the dictionaries, I find the substance of the definitions of pessimism to be (a) "Pessimism is the doctrine that this world is the worst possible." (b) Pessimism holds that the universe sprang from nothingness and is tending toward nothingness, which it may ultimately reach; this doctrine being associated with the feeling that existence is an evil, (c) "Pessimism is a tendency to look on the dark side of life and exaggerate its evils."
(a) The worst conceivable world would be one that is continually growing worse. Theosophy does not look upon our earth as such a world, but as a stage in the evolution of a part of the Universe, an evolution which tends ever to something higher and better, though not in a straight line but a spiral; that is, a cyclic evolution progressing through alternate periods of activity and rest, and on each renewal of activity after rest proceeding toward perfection onward from the point last attained. There is nothing of true progress lost during rest that has been previously gained during evolutionary activity. That cannot justly be called a pessimistic philosophy which teaches that things are growing better instead of worse.
(b) Theosophy does not hold that the universe sprang from nothingness, or that it is tending toward nothingness. Neither does it count existence an evil. Theosophy teaches that the Universe sprang from One Absolute and Eternal Reality, Spirit, one of whose aspects is Substance, whose lowest manifestation is matter. While the limitations of Spirit by matter bring about what we call evil, still these limitations are necessary for the individualization of spirit. The experiences which come through these limitations, the temptations, the struggles, the failures and the triumphs, all have their part in the development of the individual soul. These are necessary to the attainment of virtue, the growth of a strong, heroic, purified soul. The antagonism of evil is essential to the manifestation of good. We need to know darkness to thoroughly appreciate the light. The soul loses nothing that it has gained through its experiences, but carries its spoils with it. So Theosophy does not consider existence an evil, but an essential condition to the attainment of the highest good.
(c) Theosophy is not "a tendency to look on the dark side of life and exaggerate its evils", for it teaches the reign of justice. It holds that each individual has his own destiny in his own hands, that his present is the result of his past, and that his future will be what he makes it, that he reaps what he sows. This is the law of Karma. Hence no Theosophist has any reason to complain of either fate, Providence, or the law of heredity; for he has inherited according to his deserts, being drawn by a law of attraction to that parentage, that home and those conditions, for which his past lives have fitted him. So, instead of complaining, he has simply to sow the seed of good, and a harvest of good will follow; but as all humanity is linked together, he must help to lift the race in order to lift himself.
The PathTHEOSOPHICAL UNIVERSITY PRESS ONLINE