I think you should evaluate exactly why you feel this way. Your post is pretty vague. Why did you become a Catholic? Did you believe that the Catholic Church is the true Church established by Jesus Christ? Do you believe in the Real Presence? If not, what has changed? If you do believe these things how could you possibly leave? Perhaps you miss the fellowship you felt as a mormon? Perhaps there are family pressures? I haven’t seen other posts of yours so I do not know what your situation is, but I think for the purposes of this thread you should give us a little more info.
To me, the LDS church theology is very bizarre and scripturally unsupportable. For example, the Bible teaches that Jesus Christ is the same, yesterday, today and forever (Heb 13:8), yet mormon theology would have you believe otherwise. Scripture aside, the founder of the LDS, Joseph Smith, has been proven to be a fraud with the Book of Abraham/egyptian papyrus fiasco among other things.
It is a large leap to go from a monotheistic faith (Catholic) to a henothiestic faith (mormon). While mormons worship the “Heavenly Father” and believe that Jesus Christ is the “savior” of this world, they do not believe that they are one God. Thus, they are henotheistic (a form of paganism). Mormons will use the same terms as Christians -Father, Son, and Holy Spirit–but their definition of who They are differs from the Bible and from what the Early Church taught. Of course, I realize they don’t care what the early Church taught because of the supposed apostacy, which is unproven and conflicts with scripture (the gates of hell will not prevail…). And the whopper that really blows my mind is the belief that we (well, mormon men) can become gods ourselves. Do you realize that the LDS actually teaches that men (interestingly, not women) can be made gods of their own planet? I was told by a mormon that they believe this is how God makes them part of His family. Ironically, one of the very first recorded lies of Satan is that eating the fruit would make man like God.
Please post exactly why your feelings have changed and maybe we can help you. I believe there is a reason you posted here, perhaps God is trying to keep you from making a huge mistake?