I certainly don’t think that you should feel guilty if you choose to leave.
I wonder whether loneliness plays a part in your unhappiness? I have never worked for BA but as an outsider looking in, my friends who work there give me the impression it can sometimes be a solitary place to work. Being away for large periods of your life with different groups of people is not for everyone and it’s important that you feel you have people to connect and enjoy yourself with. For historical reasons I believe that the BA cabin crew might not always want to spend time with the pilots and this must be difficult.
I’m fortunate to work for an airline where the crew relationships are generally very cohesive, but I very occasionally come across a group which doesn’t want to socialise with me or each other and this can be isolating.
The best advice I can offer is to look at your pilot friends who are happiest. What are they doing differently? Who do they work for? What are they prioritising in their careers?
I hope this helps and I wish you all the best.