Many teenagers and adults, perhaps including you, have developed a fair amount of skill on a musical instrument. Perhaps they took piano lessons when they were younger, or played in the school band or orchestra. Maybe they even excelled in one of these instruments and played in a top-notch group. But even though they consider themselves to be accomplished musicians, they never learned to sight sing. They can pick up a sheet of complex orchestral music and play it without breaking a sweat, but trying to sing a simple vocal score without help makes them uncomfortable. If this is you, there is good news.
Of course, maybe you've never thought of learning to sight sing. Perhaps it never seemed important to what you do. But when you think about it, you realize that you're not a complete musician without sight singing skill. And you might live in fear of the day that someone will find you out by asking you how a particular melody line sounds. Will you be able to demonstrate it?
The good news is that you're more than halfway there. The knowledge that you have of rhythm and of music generally places you far ahead of those who are starting their sight singing training from scratch. But you've still got to bridge that gap and learn it. Thankfully, it's not rocket science; you've already mastered the hard part.
Since you're a musician, you're familiar with the concepts of a major scale and a tonic note, Do, that is the root of the scale. And you may not have thought about it this way before, but you can easily understand that, once you find Do on the staff, the other lines and spaces represent consecutive notes of the scale (unless there are accidentals).
All that is left is to learn how to hear in your mind how the other notes sound in relation to Do. This is the skill of melodic ear training, and it enables you to sing notes in any key with ease. You don't have to remember which notes are sharped or flatted, just sing within the scale.
Fortunately, most note relationships can be learned by referring to familiar songs. If we use the Do-Re-Mi system as a reference, the song "Three Blind Mice" is Mi, Re, Do. "Born Free" is Do, Sol, and can be used to find Sol, whereas Sol-Do can be illustrated by "Here Comes the Bride" or "Amazing Grace". The first note of other songs can be used to demonstrate Ti ("O Danny Boy"), Re ("Yesterday"), or La ("I Dream of Jeannie with the Light Brown Hair").
Getting used to these interval relationships and a little practice can make you into the complete musician that you always knew you could be. You'll never have to consider yourself inferior to singers in any phase of the art; you'll have it all.