Go all in. See your crush with someone else, confess your feelings to them loudly and in front of their current partner, ugly cry when it goes badly. Leave telling them you'll wait for them. Text the partner later to say "nothing personal, I'm the right person for Crush and we both know it".

Let me think, is there anything else one could do to make it more desperate and toxic? Without crossing into actual criminal harassment and stalking of course.