Hi Cosmojo,
I hope you're feeling better about your situation. I was in a similar situation when I was doing my masters, the reason I started my masters was because I didn't know what to do after graduation and I've worked with that professor before. I wasn't particularly interested in the topic but went with it anyways. That was a huge mistake, the first semester was a huge struggle for me and I constantly questioned whether or not I was cut out for it, I went through a slump for a while and finally made up my mind to quit. Like you I wasn't doing badly, but I was struggling a lot both with the work and internally. It was affecting my life so much that nothing else mattered around me and I was hurting the people who cared about me. Leaving was a really hard decision for me because I felt like I failed and I let people down. Looking back, I don't regret my decision to leave as I feel like even if I finished my masters in biomechanics I wouldn't know what to do after.
I completely agree with Chesire_Cat regarding the questions that you have to ask yourself.