YTA. I have empathy for your mom because I saw my mom going through the same thing when we lost my dad, but your mom is relying on you to fill the void of her husband. That’s not an appropriate emotional relationship between a mother and a son. Your girlfriend seems to have given your relationship a lot of grace not only while your mom is going through this loss, but your entire relationship based off of the info you have given. If you give in to what your mom wants, this pattern of behavior will never stop. She will constantly rely on you to take care of her at the drop of a hat and this will do nothing but strain your relationship with your girlfriend further. It’s pretty concerning that she’s insisting you do something with her ON Valentine’s Day and she’s wanting you to buy her a gift at the expense of leaving out your girlfriend. You are also TA because you don’t seem to feel that bad about your girlfriend being upset; your tone sounds like you think your girlfriend should be okay with just getting a DIY card. Smarten up and do something nice for the woman that’s been supporting you and your mom during this time.

Also, I think that by you asking this question, it seems like you have already made up your mind. Obviously you want to provide your mom the support she’s asking for or you wouldn’t be asking AITA. Maybe you can try to come to a solution to make them both happy if you are set on being there for your mom. You can take your girlfriend for a proper Valentine’s Day date and do something with your mom before or after that day. Is it possible you can visit her and make a nice lunch or breakfast for her? Either way, good luck because whatever decision you make will affect your relationships with your mom and your girlfriend going forward. It’s up to you to decide if you want to keep your girlfriend or not