The latin verb succēdĕre, means to follow, or to come after. It came into English as the verb, to succeed, which we still sometimes use with this original meaning. For example, we might say, “When the Queen passes away, she will be succeeded on the throne by her son, Charles.” Similarly, the noun, success, originally meant something which follows, an outcome or result, which might be good, bad, or neutral. However, about the time this play was written, success was beginning to be used in its modern sense, referring only to a positive outcome. And that seems to be the meaning of success in Titinius’ remark, “Mistrust of my success hath done this deed.” Because Cassius couldn’t trust that there’d be a positive outcome, he chose to kill himself instead.