Will my acoustic guitar sound better if I replace the strings? What are other reasons that it could sound terrible ?

Unless something has happened to the guitar or your playing it must be the strings.

I have met guitarists whose performance was improved if they just REMOVED all the strings.  They also practiced a grand total of your 15 minutes a day.  Those of us with multi- decade careers, still stay humble and forward-looking when we put in several HOURS a day, year after year. (repair of typos - sorry.)

It is somewhat fundamental, but hopefully you have it tuned properly.  Strings do make a difference, but it affect should not be as dramatic as what you describe.  There is a tuner on the computer to make certain you have the right pitch for each string.

Strings do make the guitar and the steel strings will become dead sounding with rust and dirt so they coat them somewhat and typical must change them now and then but I've found the gold coated strings from Germany will last and keep a good sound for years but they do cost $30 +....i heard one say against buying them for the cost of normal strings runs 6-10 bucks and changing them regular will always give you a new string feeling....I have a Fender that has had Gold strings on it for 6-7 years and getting to sound like i need new ones and thinking of getting off gold coated strings and try a different string thickness.... real thin strings give if a higher pitch sound sound but i don't know if it will work....i have a Ferguson guitar I bought off E-bay because it had a great looking guitar case for $100 and is has a base sound so i put real heavy thick strings on the guitar and it became easier to play and a way deeper base sound to it....so change your strings and see what your fender sounds like afterward....

Perhaps you're just forgetting that it's the strings the player manipulates and, instead, you're strumming the back of the guitar, or something. Simply check, and if you're doing it wrong just do it right. Hope this helped.