>>2806353 (OP)osaka has a lot to offer. if you list your hobbies/interests, people could give better recommendations. like a previous anon said, if at some point you dont like osaka you can easily take day trips to other cities nearby, such as kobe, nara, kyoto, and so on.
for nature, there is minoh park. this could be a half day of hiking and peaceful scenery
for a standard downtown vibe, there is the umeda district. it's mostly japanese people here with lots of shopping malls, tall buildings, and observation decks
touristy nightlife is mostly in dotonbori. it can feel like a packed theme park, but you should experience it for yourself at least once. lots of food, bars, and people from all over.
shinsekai is also touristy but worth looking at, preferably at night. it's got an older aesthetic than the previously mentioned areas.
osaka castle has big grounds you can walk around for free. it only costs money to go inside the castle. i hear the inside is not that great, so dont make that a priority. the outside area is pleasant to roam around. if you really care about japanese castles, go to himeji instead.
massive tourist attractions: universal studios and osaka aquarium. i havent been to either, but some people like them.
im sorry your gf broke up with you. you should definitely try to enjoy yourself on vacation. you may want to even cancel your osaka trip and just go somewhere else. good luck on your travels regardless.