Hi KB234 and welcome to the forums
Sorry it has taken a day to come and reply. On the forums people come and go when they can, so sometimes we don't get to it straight away. (For instant support see bottom left of page)
I myself have struggled with poor concentration, procrastination etc. I myself was ok with assignments because I think my anxiety caused me to get it done, but it took me double the time of others which was frustrating. Now not everyone is like me and sets themselves lots of time to do it (we don't always get that luxury). But maybe you could try righting down a to do list with time goals on it. So day one, intro draft, etc.
Can I ask, if you have talked to anyone about this issue? Maybe a GP, councellor or other? If not I suggest you do as they maybe able to help. At uni I know they had counselling services that were great at helping people with similar issues. You can see a counsellor for dealing with poor concentration with stressful assignments.
I am not sure if you have heard of this but every uni has a service called 'disability services' (maybe known as a different name but can enquire). They are usually attached to their counselling services (if not they'll point you in the right direction). Not sure if this applies to you, but thought I'd share anyways, as other people may read this post and relate. If you have a diagnosed mental illness (or physical condition such as diabetes) you can join disability services. All conditions are confidential. If required they will assist you with individual study plans (e.g. if you can't do full load maybe able to reduce subjects and have a bit more than part time but not quite full time etc), adjusted exams (e.g. diabetics can bring food and medication, or be seated near back of room if required e.g. bowel issues with stress) and sometimes extensions. I only got told about this service in my final year. I found it useful as for my placement they let me have half a day off for my therapy appointments.
Hope this was helpful for you.