Partner wants different sex to me.

Lying to her isn't going to help anything.

Go to a marriage counselor together. Talk about the disconnect between the two of you and ask about any outlets you can use. Yes, do this openly, in front of your wife.

A good counselor should have suggestions and, if they don't, ask about online stuff. Yes, your wife is still in the room.

She needs to know that this is an issue and there are solutions. Give her some input into your choice of solutions. You may find that she'll be okay with online stuff. She may see it as the least of all evils.

Or maybe there will be another solution which works for both of you.

Whatever happens, she'll be in the loop. Right now, she's just being deceived (because I bet when you're asked what you've been doing online, you're not telling her the truth).

You owe her this much.

Since this new kink or fetish is new to you and is causing problems with your partner, you should explore ( with your therapist) if this is related to your mental health.

Depression and anger can come out in many ways.