I can relate to a lot of what you say. I find on my better days I can feel good when I'm busy and then it feels like my negative emotions wait till I pause and then come in like a 20 ton truck.

Kudos to you for making yourself do all those constructive things which do help you feel more yourself for a while!

I don't know your age, but if someone lives with mental health problems in adolescence, it can affect how the personality develops, so we may feel all our emotions, the good and bad ones more vividly. (Has advantages and disadvantages) advantage being that maybe your activities you enjoy give more pleasure than another person with low mood would feel, and also, young people may learn how to appear well more effectively than an adult experiencing problems for the first time. (These are my theories, no more than that).

Your therapist clearly doesn't see eye to eye with you on this, but if she has helped overall, then she's probably okay.

There may not be an obvious reason for how you feel, there often isn't for me. (I often ask myself, do I need a reason for what I feel)?
I'm told some of my symptoms are down to how my personality developed living with depression as a teenager.
What I hang on to is that 'this too will pass' (how I feel now will change in a few hours or so), not easy when having self destructive thoughts, but it frequently does.

I don't want to suggest you get too introspective, but would noting your mood on a scale of 0 to 5 every couple of hours for a couple of days would show if your mood changes with what happens in the day. It won't change anything, but this fluctuation is normal for anyone, but can be more intense for some people. If you then record your mood for the day as normal at your usual time, it could let you see how the two charts relate. (Just an idea, and not a good idea long term as it could become an obsession but maybe?)? .....

A therapy group programme I have been going to has shown me that even when emotional intensity is strong, I don't need to be afraid of it. It is a symptom of the way I am, but not necessarily a sign of more problems. I compare it to someone, maybe with cerebral palsy (a developmental problem that affects movement. A person may have a stiff arm and leg. It's a difficulty they have to cope with and it makes daily life more difficult, but it's how they are made and it doesn't stop them being generally healthy and able to have their particular role and abilities in the world.

My symptoms may not be visible, but they affect my daily life, and may be in part due to my teenage development, but I can learn to manage them to become who I am in life.

I don't know if this makes any kind of sense to you, if not, no worries.
If some of it helps, I'm glad.

Best wishes for your future. ❤️