@melouise Breathe, you can do this! It's tough, really tough, but you can do it. Start in small steps, at least that's what helped with my girl who is now 11 months old.
I began by leaving her alone when she was napping in her crate (i don't use a crate anymore). Sometimes I'd have to lay with her until she fell asleep and then sneak out. Next, I'd leave the room while she ate her food, so she'd notice I wasn't there but was too interested in her food.
Then I started with being out of the house for 20 minutes (a big jump!) And gave her a nice treat to chew when I left. She was fine with that so a couple days later I tried 45 minutes. Etc etc. Slowly built up. By 5 months, she was alone for 4-5 hours at a time and free roamed most of the house.
Now at 11 months, I can leave her for however long is needed. I choose to hire a dog sitter to walk her halfway through my workday because I think it's nice for her. I do stick to a routine because if I rush out of the door unexpectedly, she can be quite loud! I tell her 'Im going now, you'll stay here and I'll be home later' and leave the radio on. With this, she's perfect and mainly just sleeps while I'm out.
I hope this helps! I really recommend you leaving the house even when you don't need to so that your pup learns it's ok. Even just pop out for a coffee at some point, gives you some me time.