Going to go against the general grain here OP.
It's your mother. If she's the one who carried and birthed you (or adopted you), who raised you, who helped pay for your upbringing and education, who was and will be there to support you emotionally, then I wouldn't be able to just bark 'no' at her and threaten to cut her off, threaten to refer her to a psychiatrist (!?! really?) etc.
Of course it varies according to your own family history and circumstances, but I know in my case it would be very hard to tell the person who has sacrificed tons for me in the past, who was presumably essential in helping me become successful, that 'the gravy train is over'.
If she is genuinely struggling with how to budget and new expenses, then you need to support her in learning to do so. The various stepchange resources people have mentioned above are great. But I'd also say that it wouldn't be surprising if she's struggling a bit, as she is ageing and we're in the middle of a pandemic that has affected many people's spending and income.
All the blanket 'just say no' answers here aren't considering your relationship, her situation, and whether she has genuine needs in an especially difficult time. Why did she spend on a new lampshade or a new set of curtains? (Maybe she's not a hoarder as posters above seem to think; maybe 9 months of lockdown staring at the curtains and lampshades she's had for the last 30 years made her think it was time for new ones?) That's more important to understand than to treat her punitively.
Also, since most of the posters here seem to have few financial issues themselves, they may not realise that it is not so easy as popping down to the building society and getting a personal loan these days, particularly if your mum is already overextended and on a fixed income. Lots of banks are increasing fees and cutting back on lending, and at a time when basic costs like fuel, food, and taxes are already going up. If she has equity she can release that's a different matter, but it depends on her personal situation as to what resources are available to her, and she may need some help negotiating those.
Perhaps introduce it as you've noticed she's having more trouble and unpredictable outgoings, so you want to help her get things sorted, which is especially important given that (presumably) she's dealing with them on a fixed income or soon will be, if she's not retired already. That's the best way you can be supportive. Especially as you are fairly fiscally savvy yourself.
If you have children, you could present it as 'I need to be saving for the childrens' educations, so let's figure out how to make sure your situation is working.'
Obviously, depending on your past relationship and circumstances - ignore all of this if it's a 'stately homes' type family, there's been emotional abuse or manipulation or whatever - it could be different.
But if it's a supportive and loving relationship, she's your mum, and think about how you can help.