Computer hardware will usually outlast the productive functionality of the software. That is to say, the software will become obsolete and unusable before the hardware breaks down.
Your 2011 MBP has reached that point now that it is fourteen (!) years old.
Both the OP and you are well past due for hardware upgrades. If secure access to the internet is important to you then it is becoming imperative that you make plans to replace your obsolete computer.
Here in the Community when one cannot afford a brand new computer, it is often recommended to take a look at Apple's Certified Refurbished Macs. You'll save a bit of money and still get a great computer will a full factory warranty.
Never buy a used Mac from eBay, Craigslist, OfferUp, or other similar sites because it's too easy to end up with a door stop when a Mac isn't prepared properly for resale.