Just trying to come up with a way to answer if guardian angels are true...okay, let's put it this way.
Angels are creatures most commonly associated with religion so am going to answer from a stand point of religion, Islam.
If we are to go by religion like I stated above, then yes, angels do really exist and are part of us even though we don't get to see them. There's a verse in the Quran which points God to saying that human should believe in the existence of angels, angels like the angels of death and his assistants. The Quran also makes us believe that angels are not just invisible creatures roaming about the world, angels are in fact divine creatures with designated task and roles which they perform individually. We hsve angels who are in charge of the rain, death, messengers and so many more.
Asides all said about Islam and angels, Christianity also happens to believe in the existence of angels and the different roles they play in the world. Take for instance Angel Gabriel who appeared to marry right before she conceived Jesus Christ. They are so many citations of such in the bible too. So yes, Angels do exist.