If you have any doubts it's never a bad thing to at least get evaluated in the ER. Having said that, had six trips to the hospital to deal with my two daughter's various injuries and mental issues. A few takeways:
Unless you're actively suicidal or planning serious harm (ie something more than typical cutting), you won't be admitted involuntarily. You may or may not be offered inpatient as a volunteer depending based what programming is available, available space, and for all I know phases of the moon.
After the second visit by my youngest daughter in a month, we were offered inpatient, but warned there might be some drawbacks- the complete lack of control as well as extracurricular instruction by the other patients. Our daughter was completely broken down at the prospect and was pleading with me not to leave her there and was promising she'd work on her issues , so I elected not to and she kept her promise and is dealing with her issues with her therapist and psychiatrist now.
On the other hand, my oldest daughter was finally offered admission for a two week program after several years of not really improving on an outpatient basis, which she accepted. Ultimately being forcibly broken of the habit by having the ability to cut taken away, the ability to put the outside life on hold to concentrate on her issues, and the constant monitoring by staff on how she was responding to her medication is what really enabled her to get better