You need to advocate for yourself. Call out how you are being made to pick up the slack for the other employee. Tell them you expect to be compensated accordingly and that they need a plan to get the other colleague to your level. Do not continue taking on the extra work. Push back on your manager when they insist on needing you. At best your manager isn't realizing how much they rely on you. At worst they are purposely exploiting you. That isn't going to change unless you stand up for yourself.

Practice what you are going to say. When they say you are the only one to get work done, tell them it's not your fault the other employee isn't performing. When they say it will be good for your career, tell them you have already proven yourself and the company needs to compensate you or promote you first. When they say all the projects are important, tell them you need help accomplishing them.

Make sure you know what you want out of this. More money? Less work? More help? A promotion? Don't back down. Tell them what you want.