Instead of the "one-itis" term, you could think of her as a long-term project and keep your options open and your life moving on, while investing alongside your interest in BarGirl.
The age difference does NOT make it sad. Where is it written that only people born on the same day could ever be together, and if that is not the case, where do you draw the line? You could be 80 and enjoying the company of people in their 20s for a variety of reasons, only one of them being sexual interest. And what's wrong in having a sexual interest in people that are in their prime? I'd say it's pretty normal and to be expected. Who doesn't like to feel young? The question is: does she have qualities other than her looks?
I personally think you should lay off her instagram for a while, at least until you're closer. Maybe she doesn't use IG to flirt with random strangers, that should be a good thing, right?
You could weigh your interest in her against the possible awkwardness of you showing too much interest and her turning you down, if you really like the bar and want to keep going there.
You could consider the option of keeping the relationship with her in the exact same stage that it's in now, without trying to win her over, just being above the average of the majority of people that come to the bar. In time (a few months to a few years) you could get closer if you still like her and all goes well (also, maybe she's in a very happy and loving relationship at the moment), but meanwhile the bar could be a place where you bring your friends, dates, etc. It's always nice to be friendly with the bar staff.
One question: what part of your life/work/hobbies/personality do you feel you can share with her to let her know a little bit about you before you make your move? I feel that the first step should be for the both of you to figure out if you really like each other.
Do you have female friends where you currently live? An idea could be to bring one of your female friends with you to the bar and show your bargirl some social proof. If you don't have female friends or feel that this is not the way to go for you, you could tell your bargirl that you don't have many friends there because you moved recently and friendzone her. This (if successful) will give you the option to show her who you are in a safe way as opposed to all the weirdos coming in the bar and hitting on her obnoxiously.
A valid seduction technique is to flirt as little as possible (even absolutely NO flirting) and just befriend people, then you can always turn things around if you find that you really like the person and they find that they really like you. The Friendzone, as we all know, is not a zone that prohibits romantic feelings. Anyway, maybe your priority after 1 month of moving to a new place should be to find people that you trust and that you can be friends with. Again, this will give you the opportunity to get to know her a little and find out if you really like more than her looks
If you don't resonate with any of the stuff I wrote above and still feel stuck and sad, another option is to actually ask her out. This way you can get some closure if she says no. You could say what you like about her in a gentlemanly and playful way, state your interest in her, ask her out, and say that if the date doesn't go well romantically, it's no pressure and you could just be friends because you feel good around her presence.
Lastly, there is one thing I feel I should mention, because you said repeatedly that you are a sad old man. My advice is to retain your dignity, that's why I said you should approach her gentlemanly. Don't put yourself in a position to drool over her and show your interest over and over with her turning you down. 40 is a good age, I can't wait to be 40, for a man it's the best age imho, a man in his 40s, his 50s, it's the time when maturity and personal style really shine. Don't try to be who you were in your 20s, there will always be an inner conflict. A man in his 40s can come alone to a bar, sit up front and drink in silence for 2 hours without being sad.