Anyway, the question I have is, what will now happen with that transaction. Is my btc lost in transit forever?
Nah, your transaction will either be revoked, pending, or completed. These are three stats in which your transaction will move. Once you will make your transaction then it will go to the pending state and after being processed by validators it will be completed.
But as of now, your transaction is pending, and either it will be revoked or completed and you can't do anything other than increase the fee using RBF features. Your transaction might be delayed longer than you expected like it might take 24 hours or more but don't worry your money is still in your wallet and will be deducted once completed and if revoked then they are usable.
One final question. As the fee is expressed in sats/vb. How do I know how many bytes my transaction is?
You can use the formula but if you would have provided the hash then it might be easy for us to calculate it for you.