Feels like Lost after Getting a Job and Working as a MERN Stack Dev

A 20 Year old, Bsc CS Grad, Working on this Job for about 2 Months. Feels like I'm underpaid, Because I'm getting 18K in hand.

Since I'm working at a product based Startup (Just team of 5, 2 of people are for sales) I have no time to invest in myself, My working hours is 8.30AM to 8.30PM.

On Sundays I have no idea work on myself, because my mind is being occupied by the tasks assigned by the Head (founder)

I always love what I'm doing, but due to unrealistic deadlines of task makes me broke, Still i manage to complete them.

I need your suggestions to improve my lifestyle (in any aspects) for example like financial, mental, professional.