I'm thinking of secretly taking money from joint savings to invest in shares
Why do you want to take this money secretly from your joint savings?
I'm going to invest it.
That's dishonest, you shouldn't take money from a joint account. If you want to invest use your own money.
I think I am going to go through with it. This just feels right.
You are essentially stealing in a way since you are not being honest about sharing the account. This is not only your money, but your partner's money as well. For this reason, she also has a say in how you guys use the money.