First of all since you just started riding maybe your budy just got an "overdose".
Next is I am thinking you should be able to make quite alot of adjustmenton your present bike. Maybe try riding the MTB a bit more. If it is much better than the hybrid maybe bring both bikes to the shop and tell them to make the hybrid as close to the MTB as possible.
Sounds like you want to ride fast and want different hand posisions. I am sure you can put dropbars on the hybrid if that is what you want.
Also if your back want a more upright ridingposition you can put swept back handlebars, wider saddle perhaps (or maybe your back want a sprung saddle).
Search for "handlebars, ridingposition, backpain" and soon. Maybe you end up on a recumbent? I`we been "leisure riding" for almost 10 yrs. This summer I tryed out an old folder, quite upright position. Did not realise how much I rode it in a short time. After some time I could hardly walk. Kicking the football for the dog made me cry out. Then I realised what was wrong and kept away from that bike some few weeks.
Do`nt worry. Read alot and see if you can borrow different bikes to see what you like best.