My understanding of that issue is that there's some cross-wiring in your brain so that parts of your brain that are usually exercised by optical nerves are also exercised by aural nerves. Pretty cool.
Anyway, with or without synesthesia you can learn to play an instrument. If you have enough rhythm to say a phone number with the same beats as other people and if you have enough tonal recognition to follow along on the ABC's song, you have enough rhythm and tonal recognition to play.
It is unfortunate that you were discouraged when you were young. But all it means is that you're behind the learning curve. It DOES NOT mean that you can't learn. Not at all.
I think you should stick with it. It's a great thing that you recognize your weaknesses going in. That gives you a huge leg up because now you know what to concentrate on in your learning. Tonal recognition and rhythm. Counting exercises will help with the rhythm. Also, use a metronome. Slowwwwwwwwwwly at first. Go as slow as you need to to be able to count rhythms. Even 30 beats a minute is not too slow if it allows you to count accurately. Speed comes from accuracy, so work on the rhythmic accuracy first. Tones - well shoot - that's a matter of intervals. Scales out the ying-yang. Learn the intervals that go together.
Not exciting stuff - scales and counting exercises - but you CAN LEARN. I am confident.
And while you're learning, enjoy the show.
- Zurf