My situation is similar in that my ds (4.5) was a very HN baby who spent 24/7 with me for the first few years of his life (SAHM in a rural area, no sitter). Here's my thing, though -- I think that being able to play by yourself/be happy with no one else around but yourself is a valuable thing, and even a bit of a skill. As such, I think that sometimes children need to be taught how to do it sometimes.
When my ds started dropping naps, I started "Quiet Time". My goal was 30 minutes in your room, by yourself. I don't care what you do, whether you read/draw/play/nap/sing/turn cartwheels...it just has to be in your room, by yourself. I did this because I think the kids do better during the day if they have a few minutes to "reset" during the day -- and so do I.
-- but also because I wanted them to learn how to simply be
by themselves without external entertainment.
It took practice -- I started with ten minutes, and when I started (~age 2.5-3), it took a lot of gentle reminders ("It's quiet time! I'll see you in a few minutes!") as I shepherded children back to their rooms. I made it fun, even, by bringing out rarely used puzzles/library books, etc., for them -- but I was insistent that everyone was going to be in their rooms, doors closed, house quiet and calm. We worked up to 30 minutes, and sometimes we even go to 45, depending on what the kids need these days.
It took awhile, but everyone caught on and now it is a beautiful thing. The kids almost look forward to it, I think, and have even initiated it on their own here and there: "I need quiet time now, Mom. Bye."
I guess my bottom line is that I really think it's a skill that can be developed and for us, it simply took a little practice. There are days where the kids don't feel like having it, for sure, but most days it is just a matter of routine. It took many months to get it down solid, but it was worth it.
I don't know whether this would work for your family, too? - but I thought it worth a mention. HTH.