Ive recently moved back home with my two cats, however we are unable to have them inside. Since being back - which is only 3 weeks, one of my cats has gone missing

The problem is, my remaining cat was more indoors than the other, and i think that she is finding it difficult adjusting to life out doors, she is 3 years old neutered and i also believe she is now lonely that her son has now gone (my missing cat) as she spends alot of time at the back door in hope to get in.
She is now bothering my neighbours meowing at their door trying to get in. I really need advise on if i should keep her and get a heater for the shed, or if I should find her a new home (which will break my heart) any suggestions welcome as I am really unsure on what do to!
Thank you