I always have this sense that my angel is standing next to me with his hand on my right shoulder. I only notice this when I think about my guardian angel but don't know if it's "all in my head" or not. Although there was one time when my angel was gone. It was during last year's Holy Thursday evening Mass. It's complicated and I won't go into the story here but I knew Hannah was distraught, by herself, and that was not a good thing at that point. I was very upset at this and told my guardian angel to go look after her. I didn't think about that again (I was at Mass after all) until a few minutes later when I suddenly thought about that "order" and realized that I felt all alone. No "hand on the shoulder" presence around at all. This was very strange ... and maybe all in my head? I kept mentally "testing" ... is he back? No. Is he here? No. About 20 minutes later, I was not alone any more. So was I grateful? Heck no! I mentally ordered him back to watch over Hannah ... and got this feeling of "she didn't need me anymore." Naturally, when I checked with Hannah later that was about the time that Tom showed up and she was fine.
So enough about me, here is what few people much smarter than I say about guardian angels. These quotes are all from In Conversation with God: Lent and Eastertide.
... all the guardian angels will gather together at the universal judgment in order to bear witness themselves to the ministry that they exercised through God's command for the salvation of each man.
St. John Chrysostom, Catena Aurea
Just as fathers, when their sons have to travel among bad and dangerous roads, make sure they are accompanied by people who can guard them and defend them from danger, so in the same way does our heavenly Father, as we set out along this path that leads to our heavenly home. He gives each one of us an angel. He does this so that, strengthened by his power and help, we may be freed from the snares cunningly set by our enemies, and may repel the terrible assaults that they make on us. He wants us to walk straight along the path with such guides, so that no obstacle placed in our way by the enemy should turn us aside from the way that leads to heaven.
Catechism of the Council of Trent
The saints intercede for men. The guardian angels not only pray for men, but they carry out duties towards them. If intercession takes place through the blessed in Heaven, through the guardian angels there is both intercession and direct intervention; they are at the same time advocates for men before God and ministers of God before men.
G. Huber, My Angel Will Go Before You
Have confidence in your guardian angel. Treat him as a lifelong friend -- that is what he is -- and he will render you a thousand services in the ordinary affairs of each day.
Saint Escriva, The Way
We must learn to speak to the angels. Turn to them now; tell your guardian angel that these spiritual waters of Lent will not flow off your soul but will go deep, because your are sorry. Ask them to take up to the Lord your good will, which, by the grace of God, has grown out of your wretchedness like a lily grown on a dunghill.
Saint Escriva, Christ is passing by