First, my condolences on your loss.
My husband opted out of European travel after a trip in 2014. Boeing retiree that he is, he finally admitted that he hated to fly and that he'd only do it in an emergency.
Like you, I always did all the planning and navigating for our trips. I think you will find that your previous experience doing that will easily cross over from planning for two to planning for one.
Since 2014, I've done one long (6-weeks) self-planned trip. All the others have been 4-6 weeks and have involved a Rick Steves tour with extra self-planned time added before, after or both. That's worked well for me.
I turn 76 next month. I hope my plans for the RS Ireland tour followed by a self-planned Wales visit next summer will work out.
Over time I've learned ...
I much prefer renting apartments to staying in hotels, largely because I like being on my own time schedule for breakfast, not a hotel's or a B&B's. But an apartment is not very practical for less than 4 nights. I've never felt uneasy in an apartment, B&B or hotel room alone.
Trips are cheaper for singles in almost every way except lodging. As you do your planning, ask about a single or senior discount and be grateful if you get one, but don't expect that it will be automatic or even happen at all.
I've never experienced any negativity due to my age or wandering around on my own. In fact, the grayer my hair gets, the better I seem to be treated. That's been true everywhere, but especially in Italy.
I've only had good train, bus, plane or ferry experiences. I pack light and I'm perfectly capable of managing my own luggage, but I am a bit slow getting my carry-on off the train or up and down long flights of stairs. I will accept help with that if the situation doesn't feel hinky. A good line for when it does is something like "Thanks, but if I can't manage this by myself, I shouldn't be here."
You said that having your husband with you made you feel "safe." I expect you'll need to be a bit more observant and aware of your surroundings and better prepared to not be a target for pickpockets than before. For me, a money belt under my clothes is essential for the really important stuff no matter where I travel.
I wear no expensive jewelry and carry no bag or purse that looks worthy of exploring. I definitely do not wear my wedding ring. It's too valuable and important to me to take a chance at losing it. I do wear a cheap ring in its place.
I always get good travel insurance, including "Emergency Evacuation and Repatriation of Remains." And I always sign up for insurance within the time limits to be covered for pre-existing conditions. I carry multiple copies of the basic details about that insurance.
I also carry multiple copies of a little emergency information list that includes contact info for my husband and my doctors, my meds and my latest bloodwork anomalies.
It probably sounds obsessive, but having that kind of information easily accessible could potentially save my life if I can't talk. The multiple copies go in my money belt, purse and a pocket in my phone case.
Have fun with your planning. That's at least half the fun for me.