Hi DDDD. I could have written some of your post. I haven't been diagnosed with BPD, but do feel quite 'off' emotionally, quite a lot (kind of unstable, with the key characteristics of emotional immaturity/ vulnerability, need for approval, etc.). I was promoted into a managerial role, didn't have the support from my management, struggled with a dual workload (was only temp promo, so had previous role also) for 2 years. Also had active undermining from those I managed. Result = near breakdown, self-esteem shot, etc. etc. I stood down from the role. Has taken guts of a year to recover, and I'm still not there.
So, that was me. What about you? If you're not yet at breaking point (in which case, yes, of course step down. It's just a J.O.B.), what about considering Cogito's second post. She sounds very grounded and sensible, with good boundaries (like wot we don't have!). I held everything in until I broke, but her stepwise points are very clear. Especially seeking advice/support of the person who initially encouraged you, and your manager's manager.
If you stick with your decision to step down, try to step down in an orderly fashion with a plan for support/training (whilst you're in your casework role) for a future go at management.
good luck with whatever you decide. I hope that you get what you want/need, and full support from your organisation.
all the best
'ender