Using a different board, a bit surprised

Hi, I've been using a 7'8" Rusty, with thruster fins. The board is pretty much a beater, its water tight, but has lots of small pressure dents, repairs, and the top of the board feels kinda hollow where my thumb grabs on the rail. I've been progressing, but thought I would be able to catch some more waves with a board with some more float, and more of a longboard shape. I thought I found a great board (pic included, and I put more wax on it a little higher up as well.), but I took it out yesterday and felt completely unskilled. My new board is a pure fun 8'2"x22"x3.125" longboard shape.
So basically I took my new board to Malibu (my first time there), and it felt way more stable when sitting, and significantly more buoyant. ..but when I would pop up I could barely ride anything, it felt like the board just wanted to tip and throw me off. This was really strange because on my rusty board, I just popped up and could ride it and turn really easy, even the first day I tried that board.
I'm confused now. I could definitely tell that it would be possible to catch waves somewhat easier with this board, but I felt so clumsy after standing up which was exactly the opposite of what expected. The board looks nice and the shaper seems like a good longboard shaper so I don't know what to think. The fins on my new board is 2+1.
The waves don't seem like they will be big enough for decent surf until Wednesday so I thought I would ask for any advice. I know it was my first day with the board but like I said, I rode waves and felt surpirisingly confident on my other board right away.
So basically I took my new board to Malibu (my first time there), and it felt way more stable when sitting, and significantly more buoyant. ..but when I would pop up I could barely ride anything, it felt like the board just wanted to tip and throw me off. This was really strange because on my rusty board, I just popped up and could ride it and turn really easy, even the first day I tried that board.
I'm confused now. I could definitely tell that it would be possible to catch waves somewhat easier with this board, but I felt so clumsy after standing up which was exactly the opposite of what expected. The board looks nice and the shaper seems like a good longboard shaper so I don't know what to think. The fins on my new board is 2+1.
The waves don't seem like they will be big enough for decent surf until Wednesday so I thought I would ask for any advice. I know it was my first day with the board but like I said, I rode waves and felt surpirisingly confident on my other board right away.