If you haven't done any maintenance since you've gotten it, it's likely that you have the following problems:

Your chain may be worn due to not having been cleaned and lubed.

Your derailleur is probably no longer indexed properly, leading to poor shifting and "hating" a certain gear.

Your wheels have gone out of true.

As for downshifting, you should downshift while you slow down. If you absolutely cannot get started again on the gear you're in, lift the back wheel up and downshift while pedaling.

I'd definitely recommend that you learn basic maintenance things for your bike. If you absolutely do not wish to do so, then an internally geared hub bike may be a good alternative for you.