he was my patient and i lost control

so i started working at this hospital a couple years ago, and right away there was this one guy that caught my eye, kevin. hes super charming and always has these big bright smiles. he came in for his checkups every few months, but i tried to keep it all professional. i thought maybe if i kept seeing him, things would just fade but they didnt. so many nights ive stared at his file, reading and rereading the same words until i memorized them. one time hes laying on my table for a checkup and hes super quiet, kind of distant looking. i ask how hes doing and he says everything is fine but his eyes looked like maybe hes not okay at all. i feel so conflicted because i do care about him, but its a nurse-patient thing right? and i have this other guy who means the world to me, were in an open relationship though... so why cant it just be platonic? after work one day, we bump into eachother outside the hospital. he looks at me with this sad smile, hes wearing all dark clothes and there are these deep circles under his eyes like hes not sleeping well. i want to hug him but i dont know what to do. the next time he comes in for his checkup, were alone in my room. hes laying on the table like usual, but hes staring at me with this look that makes my heart beat fast and my palms sweat. weve never even talked about us beyond basic stuff before now. i want to be there for him, but also im scared of crossing a line. how far is too far? whats okay in an open relationship anyway?