Bro I get you're upset but you sound super selfish rn. Celebrate some other time! Valentine's days is just a random ass date, your gf needs your support during this time and you are upset that you can't do what YOU want? Sorry but it feels like you are more interested in yourself rather than actually being there for your gf and love her the way she needs you to.

If you want a day to show off your love cool, I think that can be sweet, just sit with her and pick a date.

Oh and your mom and sister are very rude but they have a point, this memory won't disappear, it's quite traumatic and it doesn't seem like you can handle it. Doesn't mean you can't improve tho, maybe you should ask yourself why it's so important to you?

If they joke about it it might be just them trying to get over it themselves. Like convincing themselves it was not that serious and that they are OK now.

And if them keeping in contact bothers you you should ask why they are keeping them in their lives.

I'm confused, you say you care about them, don't care about the thing that happened. But you are still judging them over that thing?

So... If someone tells you about a thing that was painful for them to go through your reaction is "eugh gross"? No wonder they feel the need to defend themselves, I'd feel attacked if I was trying to be vulnerable with a guy I like and this was his reaction.

Also just because you know something is wrong doesn't mean it's easy to leave. And "I would never do that" great, congrats, they are not you tho. And you judging them by things they regret is not helping.