Did I pick the wrong bike? I live 7.2 miles from where I work and was rebuilding my life, so no car.  I was walking to work everyday until my son took me to Wal-Mart and said I was getting a bike.  I picked out a Kent 26 Bayside 7-speed cruiser.  I am 5ft 9in tall, 225 lbs and generally a big stocky guy.  Aside from taking a turn too fast in the middle of the night, and having to carry the bike 4 miles home, I have not had too much trouble, but the bike is no longer shifting to all the gears, and when in the highest gear, 7th, the chain slides off the front cog.  

I made some adjustments, not knowing what the adjustments actually did, and the derailleur ripped off the hanger.  No damage to chain or cogs, but when I replaced the derailleur, it does the same thing as the first one did.  The guys at the bike shop said I am using the wrong bike because the trails I have to take to get to work are more mountain bike trails. 

I guess, the heart of my question is, with proper maintainence and adjustment, can this bike carry me or do I really have to get another bike?  I have no clue why the chain comes off the front cog when the bike is in the higher gears, it did not used to do that, but I do know I am hard on the thing.
Whether the cruiser style bike is suitable for the 'mountain bike trails' you are riding on really depends on exactly what those trails are like. The cruiser has pretty big tires so it should be able to handle rough surfaces just fine. If you are riding those trails and it feels OK, then it's OK. 

Cheap bikes from big-box stores with Shimano 'Tourney' level components are not known for durable and reliable drive trains that will work every single day. If you want to try to make it work, learn how rear derailleurs are adjusted, it's not super complicated. Basically there are upper and lower limits that stop the derailleur pushing the chain off the sprockets, and an indexing adjustment that positions the chain correctly for each gear.

To address the issues you are having, have a bike shop check your derailleur cage alignment. The derailleur hanger can get bent which messes up how the derailleur feeds the chain onto the sprockets. This may be why you are can't shift into all gears and the chain is coming off.

Lastly, don't forget the shift cable and housing. If these get dirt inside them or damaged they can prevent proper shifting.

If you are going to get a different bike, invest in one from a proper bike shop with better components, or look for a better quality used bike.