I actually gave all the requirements that I can get, based from
this checklist. The only issue with my application was the passport, because it'll expire in April (but I'm only trying to visit for 2 months anyway, if that's of any consideration).
I am being sponsored, and the decision sounds like as if I'm going to support myself, so I think the person overlooked that. My partner feels the same way, and is very furious about it, since I gave his bank details and everything just to be neglected by the ECO.
I wasn't able to include an accomplished
sponsorship undertaking form, so I think that's why the ECO thought that I'm going to support myself. However, I included a
sponsor's letter with my application.