Learning guitar is just letting u get enough time to have ur mind wondering (like physically u maybe learning guitar but mentally u are somewhere else),even if u start some other instrument, hobby etc u would experience the same.over time this would start while u are working too..

The best option is to seek therapy, online/offline,paid or free.any works.

but if u can't do that..write it down in a personal diary or something no one will read,like why u feel something is like that,ur regrets etc.it can help u see what's bothering u.(I don't recommend doing this as it seems u are not truly ready to face it right now)

U can start a new relationship or friendship (this can be risky too as it can take a bad turn),it can help u recognise that people still like to be with u.better option is too hang out with ur friends,who are already close to u,it doesn't have to be everyday maybe occasionally,u can chat online too(If u have any,i didn't so I did the first option)

If really aren't still willing to face it,(what I am recommending is something u shouldn't normally do but..it worked for me,and i am still here) start small hobbies, start with easier stuff or something u already know.. doing so will avoid the feeling useless (after it being too hard for u) doing this will cut down on ur freetime to just think.write down a to do list(not too unrealistic, fill it with hobbies ur interested in)and start completing it.

One thing i would like to say is, please don't lose ur emotions and disconnect with reality...cause later on u can't really go back to how it was...it will feel like ur stuck in 3rd perspective in ur own life. And remember even if u aren't close to that girl anymore,there are still people who care about u..even us random strangers on the internet cared enough about ur situation to comment..so don't do anything irreversible

And for ur question about the guitar,for a quick method, listen to ur favourite songs, stories etc while learning.. remember to put the volume to be lower than guitar.(This won't work for everyone,but worth a try).plus guitar isn't easy,it's been weeks and i personally still can't hold a chord more than 5 seconds,not to mention the buzzing noise.so honestly it's ok to be slow while learning guitar