A very natural worry, but
Mums, esp bf mums, do develop a sense of where the baby is in their sleep. Think about the last time you accidentally rolled out of bed in your sleep beacuse you didn't know where the edge was? Never? It's a bit like that but more acute.
I co-sleep out of necessity (velcro baby like yours) and my tiny dd sleeps sort of "under my wing", I have one arm stretched out above her head so she can't migrate upwards, and my leg bent underneath her (I mean further down the bed, not that she is on top of me IYSWIM) so there is nowhere for her to shuffle to. She is snuggled into my side usually so that I can feel her at all times, and very often my nipple is still in her mouth - hard to detach sometimes without waking her!
VERY important not to co-sleep if you don't tick the guideline boxes re smoking / drinking / drugs / v overweight (I think, someone correct me on that one if I'm wrong) or very exhausted. Also not adviseable for prem babies.
I found it took a few weeks before I really relaxed and trusted myself. Possibly the worry played a useful role in the early days to ensure I learned how to perfect the art!
Co sleeping is the norm around the world, esp places like Japan (lowest SIDS rate in the world) and after all, it's how we evolved.
Co sleeping saved my life, and has played an enormous role in keeping the bf going.
Good luck, and congratulations on your baby!