2:1 grades at Oxford are good. Averaging a good 2:1 and high 2:1 results is definitely good. A 2:1 is not an "average" grade and the fact you think so does not change that. You need to re-calibrate what you think are "good" vs "average" results in uni and realise it's not like school where anything less than an A* is "average" (which is not true in a school context either but I think more likely than not you are one of those types that thought that).
Also you need to re-align your expectations to those of others - specifically employers. For working as a solicitor a 1st isn't going to make a difference anyway, you have a 2:1 or above you tick that box on the recruiter's sheet and they start looking at other things in your application. You should however be looking at getting work experience/vacation schemes etc during the breaks between terms wherever possible. As that will make a difference, and not having that is going to be a major barrier.
There's also no reason to wait before pursuing any kind of work experience - even short shadowing opportunities, doing some more basic work in the court systems etc to demonstrate more familiarity with and engagement in your planned career. Plus, crucially, being able to demonstrate on paper that you did the work experience and therefore are actually capable of turning up on time every day and doing what is tasked to you!
In any event, whether you wish to see a therapist or not is up to you, and wouldn't have really been my first thought anyway. You may of course want to discuss with your GP and they may be able to make some suggestions although ultimately it's up to you whether you want to follow up on any of those. But you should definitely speak with your personal tutor about things, as they can provide some much more insightful comments on the situation and also should be able to contextualise your performance well so that you can understand you are doing well, as it will be someone who knows better than anyone if you are or not.
Considering dropping out because you're "only" getting good to high 2:1 grades is one of the most patently ridiculous things I've read on this website though and really speaks to the depths of your cognitive biases in how you are seeing things from a very warped perspective though. Which is something you need to address one way or another.