I agree with other people's suggestions above. I retired from full-time working last August (stayed part-time as they still needed some of my skills), but well before I left, I drew up a spreadsheet of what I regarded as obligatory outgoings (mortgage, heat and light, insurances, etc) and totted it up, to make sure that my pension income would cover this. I then cancelled anything I didn't consider necessary - some of the savings were from cancelling professional subscriptions which my work didn't reimburse. I now keep that spreadsheet going, with income and these expenses, each month, to make sure I stay within budget. With the lump sum from starting my works pension, I was able to pay off the mortgage, which made a huge difference. I was also due a lump sum option on my state pension as I'd deferred it as I was retiring later than retirement age.
The drop's not yet devastating, I still have a good pensionable income (partly due to deferring the state pension where you get an extra 10.4% for every year you defer), but I'm sure it will bite if unexpected costs come along (e.g. I had to get a new battery for the car last week).
Forward planning is absolutely vital, so you know what will be coming in, and how much you need to cover all your essential outgoings.