I'd say that we're a massive country that should be able to walk and chew gum at the same time. We could (and should) address what's going on at the southern border, but that doesn't mean that we shouldn't also provide aid to Ukraine. It's not like we can't do both.
I'd also argue that the war in Ukraine is very much our problem. We want this act of aggression to hurt Putin and Russia badly, so that other power hungry countries think twice before invading a sovereign nation, such as China invading Taiwan. If the Chinese invaded Taiwan it would be really really really bad and would directly affect us in a significant way. If the support we give to Ukraine continues to make Russia's lives miserable for this act of aggression, I think it's well worth it and might prevent further aggression down the line.