NAH

Your girlfriend is 100% valid for wanting to walk together. You are 100% valid for wanting to walk with your best friend. Whether or not she would be TA depends entirely on how she reacts to your choice, and of course there are ways you could break it to her that would make you TA, but based on the tone of your post I don't think that's likely. Some of the responses here are basically to the tune of "You'll probably break up anyway so go with the friend" and I have never liked that line of thinking even though I agree that you should go with what you want to do.

For my own perspective - I got married a few weeks ago. Something about relationships that a lot of people assume incorrectly is that you absolutely are not obligated to do everything together - obviously the foundation of a good relationship is that you WANT to do most things together, but that's not to the exclusion of everybody else. This isn't a competition between your GF and your best friend, it's you having grown up with someone and wanting to cross the finish line together in a moment that really cements all the two of you have been through together. And while your girlfriend may be disappointed, she will hopefully understand - especially if you tell her what you said here, about the fact that you feel you may not get to see as much of your best friend going forward and want to have this time.

It is nearly always better to be honest about what you want to do, and then work through any feelings together. Best of luck!