I think I might be slipping into depression. How to prevent it?

The unfortunate news here is that... You can't. Everyone is on their own journey and you can fight, struggle, do 10000 different things to put a bandaid on a festering wound and it will just slow the healing. Even therapy can be not effective. Radical acceptance is the way. Accept your feelings, don't try to run away from them - you will be wasting years. While acceptance deals with the discomfort much faster and letting yourself feel bad is part of life. It's a natural feeling. The question here is to ask "what is it telling me and what should I change about my lifestyle to experience it less". But it wont go away completely, it's a part of our behaviour that's purpose is to show us that changes in our life are necessary to be made.