I completely understand how you feel. One of my favorite kiddos is leaving next week - literally the day he turns 3 to start a federally funded pre-k program. Do I feel it's in his best interested? No. Do I think he'll be getting something he doesn't already get with me? No. But I'm not his mother and his parents are the type that they feel the need to push every milestone. This same kid has been pressured to potty train for over a year and guess who is not remotely close to potty training?
Unfortunately this is the hardest part of the job for me because I, too, get more attached than I probably should.