You absolutely can change your situation. You do not have to stay miserable or stuck or permanently unsatisfied. It can be daunting, when you have a goal that seems impossible, but you must have faith in your own decision to want to get there.
Tim Ferriss wrote:
"The decent method you follow is better than the perfect method you quit."
An academic advisor back in college once gave me this sage advice:
Q: How do you eat a whale?
A: One bite at a time.
Start with one thing. Cut out bread. Add another. Walk one morning per week. Make a decision to slowly replace bad habits. Build upon them. Keep doing things for your health that are slightly better than the things you did yesterday. Allow yourself failures. Recover. Repeat.
I'm writing this for me, but feel free to use it for you. We pretend not to know how to be happy, but a healthy body and mind do seem to make the quest a lot less daunting.