First I would see someone and get Trackman numbers to find out the characteristics of your swing then go from there.

There are tons of low spinning shafts but we really need to understand your driver numbers to provide a recco.

Do you know your launch and spin numbers? Can you get them?

Trackman would be your best bet

The short and best answer will be to find a fitter. A solution may as simple setting the loft lower on a 910-917 driver, a small change in swing, or a new head loft and/or shaft replacement. I would consider a $75 fitting an insurance policy on getting the correct $200 shaft the first time - if that is the best solution. Depending on what version head, it could even be a better investment to get a whole driver that will likely waive the fitting fee, allow you to sell the old one and not be great deal more - net - than a new shaft.

J
I have the same problem, went to a Titleist Thursday fitting session and after checking my numbers on Trackman and trying various set up I was recommended a 917 D2 (8.5 degree) SureFit Setting: C-2, with an Aldila Rogue M"AX 65 (Regular)
This works for me far better flight and consistency, might be worth you going to a Titleist fitting session, they can recommend a set up that suits you, that may be transferable to your 915.
If you're looking for just a shaft upgrade and are unable to get to a fitter, I can recommend a Grafalloy red shaft, they're designed for stronger swingers, the Grafalloy Red shaft delivers a boring, penetrating trajectory via a stiff-tip, highly stable design. I have one fitted to my 917 F as the Diamana stock shaft was for me uncontrollable, hitting high right and lacking in distance.
I've also used this shaft over the years in other drivers and 3 woods and it really does help to keep the ball flight low.