Lost all legacy benefits

Good Morning

It’s been a while since I’ve posted on benefits and work.
Would like to ask a few questions.

A while back in march I changed my job as I was struggling with my hands and previously had plate fitted in toe and struggle day to day.
Am in receipt of PIP. Plus review now ongoing for 11 months so more anxiety in the wait of this.

I was claiming a legacy benefit tax credits and the disability element plus the severe disability payment in housing benefit/council tax reduction.
Because of changing my little part time job I lost my tax credits I then put a claim In for UC this put a stop to my housing benefit ,and severe disability premiums top up , I became a wreck through having no income as couldn’t cope at work through the stress of this all , i got my job back by the skin of my teeth but got to prove I can work without no more sick days.
Only have 4.9 years before I retire if I ever get there.

I contacted UC customer services how I go about getting my severe disability premiums back as I was claiming this before UC before , every time I phone UC I’m told to put it in my journal, was told we will phone you , we will contact you. so today I received a call that they are looking into this , instead of letting me stew over this for weeks why not contact me. All it’s caused is undue stress.

Would I be eligible to get this back ? And if I don’t win this payment back will I be able to ask for a mandatory reconsideration ?
Deep down I’m thinking it’s a no win as it’s taken too long for a callback.

I didn’t want to ask too many questions or speak as to say the wrong word.

Hope I haven’t gone on too much and get some answers from DWP and maybe from benefits and work.