When is my trademark too similar to a competitor's mark?

In general, the law of trademarks provides that the first company to secure a trademark can preclude others from using on the same or related goods or services the same or a simiilar trademark if it is likely to confuse prospective customers as to the source or sponsorship of the goods or services.

Note that trademark law developed before there was global marketplace, and that each jurisdiction applies its own specific principles as to when and how a trademark is secured. Thus two different companies serving different geographic markets might have independently adopted and secured valid rights to the same or very similar trademarks for the same or similar kinds of goods.

In the United States, there happen to be various ways to secure trademark rights. The gold standard is to register the trademark with the U.S. Patent and Trademark Office. But it's also possible under U.S. law to develop rights in a trademark just by putting it on the company's goods and selling them widely, without registering it. This kind of right arises under the "common law." And to complicate the matter even further, each State of the U.S. separately accepts registrations of trademarks, typically where the registrant is planning relatively local use of the trademark.

Trademark rights in a majority of countries other than the U.S.A. typically are secured by registration with the government. And the standards applied in various jurisdictions for determining "how close is too close" aren't entirely uniform.

To answer your question, the first step would be to engage an attorney with experience in trademarks and seek an opinion on the availability of your proposed trademark in the jurisdiction(s) you expect to market in.