A mental prison is an apt metaphor for crippling shyness. I agree with my colleagues but only if the shyness is crippling. Most people have some times or circumstances in which they are not as comfortable as they'd like. While this may be defined as shyness, it is not crippling; it is more of a stumbling block. When the shyness prevents normal activity or behavior, it can certainly be as confining and restrictive as a prison. Agoraphobia, in its severest forms, is an example of this phenomenon.
Shyness can be a mental prison. If you allow the shyness to prevent you from building new relationships and engaging in new experiences. However, you can put a mask over that shyness. You can develop a persona that hides the shyness and function outside of the shyness even though it still exists.
The above posts are right on target. Shyness can be viewed as a mental prison in that it can confine you to your own place in the world. It can prevent you from communicating with people around you and often causes people to miss out on valuable opportunities as well.
Shyness can be a crippling condition, in that it affects every social aspect of a person's life, from how they interact with their family, their co-workers, and potential mates. It affects a person's confidence, education and even their economic situation. As a mental prison, it confines the person in a social straitjacket of sorts. It obscures who they are from the world. In these ways, you could consider shyness a mental prison.
You might want to consider various literary characters to give you some insight into this phenomenon. When I think of shyness, the character that stands out most to me is Laura in The Glass Menagerie by Tennessee Williams. She is a girl who is literally crippled by her shyness, and in a sense her limp is an outward manifestation of the impact of shyness upon her body. She is emprisoned because of her shyness and is unable to participate in the world.
A prison is a place where one is confined. A mental prison means you are confined to your own mind. Shyness keeps you from easily interacting with new people, you remain stuck with your own thoughts and feelings because you are uncomfortable expressing them (confined to your own mind). You also miss out on the thoughts and feelings that might be relayed to you in these interactions. Again, confinement separates us, this mental prison does also.
Shyness is caused by many reasons, but more often than not, it is because of a lack of feeling like one belongs or a crippling belief that one will be rejected for whatever reason (looks, economic status, intellect, socially inept, etc.). This will inhibit the shy person from acting on impulses to follow his or her dreams and pursue goals. Being shy can be every bit as crippling as being physically handicapped or deformed. It limits one's abilities somewhat. This can definitely be considered prison-like.
Being shy, like being in prison, keeps you from going where you want to go and doing what you want to do. A shy person may want to participate in life, be part of a group, have relationships, share opinions, and express emotions, but can't do so because he or she is imprisoned by fear. Just as being in prison, being shy prevents you from traveling in a wider world and exercising real freedom of choice in action and expression of self. Shyness is a mental prison, and it's very difficult to break out of it. Escaping requires great determination and sometimes help from others.
If you think about it, being shy can be something like being in prison. In both cases, a person is walled off from the sort of contact with others that is easily available to "normal" people.
If you are shy, you are likely to have a hard time dealing with new people and new situations. That will make you withdraw to some extent from meeting and interacting with others. If you withdraw, you deprive yourself of contact with other people. If you are deprived of contact with others, it is similar to being in prison because prisoners (especially those in solitary confinement) are unable to interact in normal ways with other people.
Shyness could be looked at an a mental prison if it is holding the person back from making healthy relationships. This then causes students to often spend too much time alone overanalyzing their faults. They often get more introverted and over time their shyness gets worse. As they get older this causes lifelong relationships issues and relationships in the work place. All of this could eventually lead to depression. It first started out as what seemed like simple shyness but turns into a cycle that is hard to break.