Really Upset and Lost on what to do 😔

I am so upset, depressed, and at a true loss on what to do.

I have a 5 month old Shih Tzu and a 13 year old Maltipoo.
They aren't friendly, the Shih Tzu bullies my Maltipoo constantly
So far it's been aggressive, but not unmanageable. A few nips, biting of the tail etc

But, this morning it went way too far. The Shih Tzu bit my Maltipoo on the neck. And it wasn't a nip, it drew blood.

I'm devastated and horrified. I was watching them, the Maltipoo had a toy, Shih Tzu wanted it and bit his neck to get it. You can't confuse an animal's neck for a toy/mouth, the Shih Tzu isn't that dumb; leading me to conclude it was on purpose.

I am crushed. I don't want to give the Shih Tzu away. I love him already. But, I can't put my Maltipoo in imminent danger. The Shih Tzu needs personalized training, I know that. I am working on how to figure that into my budget.

In the meantime I really could use some honest, helpful advice. I don't want "advice" of people telling me I should have never gotten a puppy, I already feel terrible enough. Thanks in advance.