First of all, OP, you're not useless. If you were, you wouldn't be feeling the need for change. Try to be kinder to yourself. 
Second of all, I could have written so many bits of your post word for word about where I am at right now, so I really get it. I too am feeling really p'd off with myself atm for being less productive than I want and need to be (no work atm, self-employed and it's dried up thanks for COVID), my MH not good (anxiety here too, couple with depression), letting my sleep patterns slip and struggling to motivate myself to shower some days. I'm trying to remind myself that these are frightening, unprecedented times and even those who don't have health challenges to face are finding this whole thing tough going.
I've been thinking about this a lot, about how to simultaneously be kinder to myself but at the same time give myself a metaphorical (gentle) boot up the arse towards where I really want to be at the moment... can I hazard a guess at something, though please feel free to tell me to bog off if I'm completely wrong? 
The way you talk about self-sabotage and the promising things you have ahead of you (dissertation, job offer)... do you think it's possible that having got so close to where you want to be in your career, you might have grown frightened by the possibility of 'falling at the last hurdle' so to speak and so you're starting to back-track a bit?
I might be talking bollocks here but the reason I ask is the situation I am in right now... when not looking for work, I am currently writing a novel. I have been writing it for a long time. it is generally going well, I feel I can pull this off, if I manage it I will have fulfilled a HUGE dream of mine that I've had for about 35 years, that have been spent learning to write creatively, and practising like crazy, and writing a lot of absolute shit while trying to find my voice , and finally I've reached a point where I feel I can do this. And yet... right when the next step is to get it finished and 'smash it over the net' so speak, my doubts creep in, and make me doubt whether I really can do this after all, and this inevitably leads to me sabotaging myself by wasting a lot of the time I could be spending writing on sitting around doing much less meaningful things instead... watching trash TV, letting social media swallow up too much of my time, succumbing to sleep when it gets too hard to work out what my next productive action should be (I have a sleep disorder, so the struggle to resist daytime naps is VERY real ) Because there are times when putting myself to the test simply feel too scary, in case I discover I can't hack it after all, and the mere thought is enough to make me want to shut my laptop and start watching the Steph Show or something.
Sorry, this may seem like it's descended into an irrelevant ramble about myself but the point I'm trying to make is this: do you think it's possible that you feel stuck, and are prone to self-sabotage, because of worries about the next steps, about the opportunities they'll present, because the stakes are so high now?
I'm doing my best to work through my own self-sabotaging tendencies as best I can, but with anxiety we have to remember to take it slow and be kind to ourselves as well. I like @springydaff's suggestions about taking things a bit at a time, making a plan but not beating oneself up if the plan has to change.
Hope that ramble has been of some help. You'll get there, just take it one day/activity at a time, give it your best shot, but don't beat yourself up if you have an off day. You can do this.