Hey Danielle,
I have a 2 1/2 year old son who also slept better as an infant than he does now. His sleep started changing around the 2 year old mark, and since then he goes through spurts of sleeping through the night and waking several times a night. From talking with other moms of slightly older kids, they've experienced the same thing. My gathering is that this is totally normal for this stage in their lives. For me, at this point, it's often due to my son having dreams that wake him up. As their minds grow, so do their imaginations. What I'm trying to say is that for now it's possible that sleep habits are going to be changing regularly and you really just have to switch it up and find something new that works... maybe even on a nightly basis. Here are a couple of ideas that we have done to transition to this new phase:
Let him fall asleep in our bed, then transfer him to his bed.
Sit on the floor in his room with my back to him so he'd see I was still close.
Bought a HoMedics sound machine / projecting night light at Linens N Things (this has really been great).
Walk him back to his room at night (even if he opposes this idea) and if need be, sleep in his bed so he knows this is where he is supposed to sleep.
I hope some of these ideas help you. Sleep and I are best friends, so I understand your frustration!
Take care,
M.