You suspect that your fiancée may be using you for a place to keep her pets and her things. That is a realistic possibility. According to your letter, she has been spending time with another man, lying to you about their time together, has informed you that she loves him, and is not sure who she wants to be with. The only reason she may still be living in your home is because the other man may not be able to accommodate her pets and her things at this time.
It is difficult to determine if she is using you but all signs point to that conclusion. I have limited information about your relationship but it seems as though your fiancée is preparing to leave you. I cannot say that with certainty but with the information provided it seems to be a realistic possibility.
I am also concerned about your feelings. Why should this be her decision? Why would you want to be with a person that has lied to you about spending the night with another man? She has in essence been cheating on you and has done so in a veil of lies. At this point you need to ask yourself if you really want to be with a person who is capable of treating you this way. She is taking the time to decide if she wants you. You are the grieved party; you should be the person deciding if you want her. There are many woman in this world who would not cheat on you. They might end a relationship but they are simply above cheating. When they have serious doubts about their love, they will end a relationship rather than cheat.
I’m sorry that this is happening to you. Please take care.