I have almost zero experience in this area.. but I'm currently fostering a 10-year old dog who hadn't been spayed. She had been dumped in a pound with a false pregnancy, various growths, including tumours in her mammary glands (50% likely to be cancerous) and the lady running the charity said it's very common in mature dogs which haven't been spayed. You could do a search on mammary lumps in dogs to read up a bit more.
https://www.acvs.org/small-animal/mammary-tumors
My wee charge had quite a lot of surgery on Wednesday (they extracted 36 teeth, removed tumours and growths and spayed her. Added to that has been medication for a false pregnancy - again a result of her not being spayed - and antibiotics for a badly infected mammary gland. I imagine the medical bill will be in four figures by the end. Had the charity realised she had tumours, they'd not have agreed to take her and the pound where she'd been left would have put her down.
After anaesthetics, dogs are poorly anyway, so the first 2 days were horrible: she was nauseous, her temperature was plummeting easily, and she needed a lot of encouragement to drink or even lick any food - so giving medication took patience. Nevertheless she was really happy to go around the block on the evening of the second day, and thereafter. Five days later, my partner has just given her a ten minute walk around the garden, and on return she went straight to the front door wanting a longer walk outside. She's still in pain, and is sleeping or just lying most of the time, but remember that it wasn't just a simple spaying - 36 extractions and removal of lumps, bumps and a couple of mammary glands. Lots of little walks, dog TV, tastier food than usual and plenty of love will always help a wee dog to be happy. She's perking up more quickly than I'd anticipated. I would just advocate that if you get your dog spayed, you should take time off work and cancel any social commitments, so you can safeguard and care for your dog in the first ten days of recovery.
My opinion, to judge by this experience, is that spaying is horrible, but the worst part is the recovery from the anaesthetic.. By getting her spayed, you're probably helping her to avoid anaesthetics, pain, upset, medication and pricey medical bills when she's older, and possibly giving her a much better chance at life if she does end up changing hands for any reason.
I hope this wasn't too detailed! Good luck with making your decision. Don't worry about your dog blaming you.. As long as you're there to tend her during her recovery, the bond will be just as strong. She'll probably hate going to the vet though.
Like the other poster, having had this experience with this lovely little dog, I'd definitely have a dog of mine spayed at the earliest possibility. Dogs tend to get more sedate as they age anyway, but I think their behaviour is a lot to do with their temperament, and she's not going to lose her personality and spirit just because she got spayed.