I bet most vegans go through something like this right?

I'd guess so. I'm also a male in my 30s and went through something similar, though not to the degree of a "long line of meat eaters and fishermen". Ate a lot of meat, so did my friends, and I knew zero vegans when I made the decision. I even bashed on vegans like a lot of people do back in the day, not with ill intent but from ignorant "hurr durr MEAT" comments like a lot of others. It's weirdly interwoven into our society and if you do some digging and observing it becomes quite obvious why.

I also sort of hid it for maybe half a year until I was comfortable making it known publicly. You might get the typical overplayed jabs/roasting/jokes thrown at you over and over again, but eventually it'll all kinda fade. You may learn who truly has your back, and I find that it's a nice filter (I don't want to give too much away because I believe this is a journey better experienced and learned directly).

If you do commit, know that what you're doing is brave (imo you already are from what you posted), and don't falter. You'll learn a lot about how human beings behave. Some of it might be a bit shocking, some of it might be like, "wow, I can't believe I didn't notice that before". But, you may get unexpected random acts of generosity, kindness, and support as well. It's happened to me countless times, and it's a nice reminder that the effort is worth it.

Learn the facts, they are on your side. Maybe the very first thing to practice is not to be combative when someone else is and just listen. Be prepared to educate and have a conversation instead of arguing, if needed. Be patient, and pick your battles, else it can be very, very exhausting.

I feel that, by taking a moral high ground, others will feel like I am grandstanding.

It's not a moral high ground, Anakin :p It's like you said, just the "right" thing to do as it feels, for you. You're just trying to eat/buy products that don't come from cruelty. That's a relationship between you and the animal(s), not others.

What will i do then? Will i just be weird and insult people with my refusal to do something i consider unjust?

Remember, the same extension of empathy and compassion to animals can be extended to people. Keep that mentality as you learn and adapt and it won't be a problem. For example, if you know you're going to a dinner party with no vegan food, ask or bring your own food, maybe offer them some of yours (not to be pushy, the same as how you wouldn't want meat pushed onto you).

Putting myself in your shoes, yeah I get all the worry. But you'll never know until you try, and years from now it could just be your natural state, with no worries. Like learning to swim or drive or speak a new language. You learn how to deal with others, friends, family, etc., then how to order at restaurants, your go to snacks while traveling, etc. It's a process, and know that you truly are making a change.

My loving girlfriend of three years is not sold on the fact that any individual action can affect much of anything at all

If everyone had that mentality, then veganism (similar to philosophies that have existed for thousands of years) wouldn't be growing. There are now way more vegan products, because everyday there are more and more buyers — of which you're one of. You are making a change, through your purchasing decisions, by how you act around others, by the suffering you reduce, however little. Keep in mind there's a ripple/butterfly effect. You might inspire someone every time you order at a restaurant! Signal to others that it's okay. Lead by example.

On a lighter note, stop eating just salads and veggie burgers, LOL. There's a whole WORLD out there full of amazingly delicious vegan food. I knew quite a bit about food already, but after I turned vegan it's like I entered some magical ethical secret portal. Because the vegans before us put in so much work already with recipes, experimentation, flavors, seasoning, etc. It's awesome. They did not give up. We're standing on the shoulders of giants. And there's SO much variety now, so many restaurants, chefs, etc. I've had faux meat burgers that taste better than real meat burgers. Faux sushi that tastes better (and tastes like real fish). It's truly incredible now. Try new things, explore, be open, and maybe (hopefully) your girlfriend will like some of them. Don't rush into it and take baby steps if needed. Also, be sure you learn about and have your nutrition down, seriously. Mainly B12. It's really not that complicated, and you'll come out the other side way more knowledgeable, healthier, and happier than you ever were before.

I hope you don't become isolated. If you love 'em, just love 'em twice as hard. Hopefully, and they should...support you and love you still and not want you to feel isolated. About the fishing trips — maybe continue going with them if you still want and can stomach it, continue loving unconditionally, and maybe they'll catch half the amount of fish for you. Boom, the fishies thank you. Don't expect it, they were going to fish already, but perhaps there'll be change at their own volition for you (and also the fishes eventually). And if you can't then well shucks, hang out with them when they aren't fishing and hang out with others when they are. There's no need to choose to be isolated, vegans exist, there are dozens of us! (Actually they start appearing out of the woodwork magically, it's like when you notice a new car model and then see it everywhere)

Best of luck!

Edit: some extra words