You need to set boundaries. I know that you are caught in this cycle and don't want to upset your mother. But what is the worse thing can happen? So she finds out you are going to a therapist. She doesn't approve and doesn't think you should be spending your money on it. So what? You work, she doesn't. You need a neutral 3rd party to work out these issues and set boundaries. You are in the prime of your life and you are squandering it away because you are afraid of upsetting your mom. You will never get this time back. Go back to the gym. It's good for your body and social life. Maybe you will get back with your ex and maybe not, but at least you will reconnect with friends.

Seriously you have every right to date who you want. You have every right to some privacy and a sex life and not being treated like you are 14. Your mom has serious issues. Don't make them your issues. You are not responsible for her happiness. She will get over you living your own life. You need to separate your living situation as soon as possible.

Do not give her a key. Hopefully a therapist can help you to say "mom, I love you, I want you in my life and I will invite you over when I'm home and we can visit. But you cannot have free access to every detail of my life. It doesn't mean I love you any less. It just means I'm an adult and have the right to my privacy."

Seriously one day your mom will be gone and then you will be "free". However, if that doesn't happen for 20 + years and you haven't set boundaries and started living your own life you will look back with huge regrets. You may be a 60 year old who can finally start living your own life, but why wait?

Please please please invest in seeing a therapist. It's the first step and the best investment you can make in yourself. Your relationship with your mom is not healthy in any way shape or form.