You may use your own relay as a guard only if your relay is accepted as a guard in Tor network. Not all relays will be accepted as a guard in Tor network. It needs to be stable relay first, if a relay does not get the stable status first, it will never become a guard (entry node). However, being stable relay is necessary, but not sufficient to become a guard. The relay should also be fast enough to become a relay after some waiting period. The definition of fast enough will change from time to time, but currently, I believe the relay should be among top 25% of all relays in term of speed to have a chance to become a relay after some waiting period. I believe this waiting period to be about 90 days at this time.
Now, once your relay get the status of being guard, you may think about if you like to use it as your own guard. Technically, it may have some advantages to use your own relay which has obtained the status of being guard in Tor network as your own guard because it may mask your own traffic. But do not jump in to conclusion too fast, this will all come to nothing if there is a malware in your system. Read this question for more information: Equation: The Death Star of Malware Galaxy, How much does it compromise Tails?