Cognitive Behaviour Therapy is evidence based and has reasonably good effectiveness - about 60% of people going through it experience good recovery.

https://www.nhs.uk/conditions/cognitive-behavioural-therapy-...

One good thing about it is that it only lasts about 12 to 16 weeks.

Find someone with qualifications, experience, and a registration. Check the pricing over 21 weeks (this is the maximum you'd possibly use, but you're likely to use fewer lessons.) Then book in. Don't be afraid to change therapist if the one you have is terrible.

The first session will talk about the CBT model, and ask you what you want to achieve, and talk about confidentiality and payment.

You'll need to be able to identify emotions and hot thoughts, but the therapy should help you with that.

EDIT: a lot of people are recommending exercise. It's important to exercise for your physical health. The evidence for benefits to mental health is somewhat weaker, and we've only recently started getting good quality evidence that it works. For you it may work, it may not, and it's probably not going to hurt to try unless the failure of exercise to lift your mood is going to feed into negative thinking.