Solipsism is the belief, usually quite brief, that the thinker is the source of everything that is - my feeling that everything is just a product of my own thinking and has no independent existence: my life is a dream.
Solipsism seems to me a state which we must go-through during our spiritual development towards divinity - because it represents the exact point at which we become wholly 'free': the point at which we become fully agents, and detached from the causes of everything else which impinges upon us.
We are conceived and born, unselfconsciously immersed in the universal world which we simply accept; but the moment of solipsism is when we become independent of that world.
Solipsism is therefore - momentarily - necessary, if we are to move from being immersed in the world to a situation in which we engage with the world from a position of free agency.
If we are to become grown-up children of God, at some point our selves (or 'souls') need to become god-like - which means that our relationship to universal reality but alter profoundly from immersion-in to engagement-with - we need to have a relationship with reality that is voluntary, agent, purposive, and conscious of itself.
To be fully free we must be fully conscious - which means that we must know what we are doing. Sadly, this means leaving-behind the un-self-consciousness of childhood and of early stages in our cultural history; and it really is left-behind - because the process of detachment signalled by solipsism is irreversible.
We have grown a shell, and breaking that shell (voluntarily or with with drugs, or by disease perhaps) is not a return to innocence, but some kind of pathology.
We can really only go forward - indeed we must go forward because if we get stuck in solipsism - as so many modern people seem to have chosen to do - then nihilism and despair are inevitable. In solipsism we begin by regarding the world as our own thought, but soon (and inevitably) we begin to doubt the reality of these thoughts - after all, thoughts change, they are not solid...
The self in solipsism surveys the world paralysed by doubt - the thoughts are transient, the world a product merely of thoughts - everything slips away. The self doubts its own reality... (The situation was depicted many decades ago in the world of Samuel Beckett.)
In solipsism the mind thus alternates between an assertion that the self is the only reality, and the recognition that if this is true then there is no self.
Thus solipsism is a necessary and inevitable phase in spiritual development - but ideally it should occupy only a very short time, a full recognition as brief as possible - we should go through solipsism to a new, free-and-agent relationship with universal reality - a state that is qualitatively the divine relation with reality.