You can replace internals on your shower valve. Im not sure what brand that valve is but…first shut off your water at the main. Open your hot and cold side on your sink faucet. Go outside and open a spigot. This will drain down your pipes. Underneath the handle will probably be a tint set screw which is probably a small allen screw. Loosen this and remove the handle. Then there may be a decorative ring over the valve body (an escutcheon). Remove this if it has one. There will either be a threaded ring or the whole valve body will rotate. Unthread this assembly. Now the guts of the valve will be in your hand. Typically youre going to find 2 small black seals held in place by 2 small springs. These being worn out is whats causing the leak. They sell replacement seal and spring sets at the plumbing repair section at you local orange big box store. Reverse procedure after installing new springs and seals. Its helpful to know the brand and model of your shower valve to get the right seals. If not, take the old ones in and match them up. Id consider this to be a homeowner friendly repair and if you get confused or shit looks weird put shit back together and call a professional. Discretion is the better part of valor.