What a heartfelt message, and i can say, as a fellow single Mum, that i know exactly what you mean and how you feel. Without question, weekends and evenings are the most difficult.
I am a little further down the road than you (dd is 8.9), and I found that what worked best (without question), was to always have a plan for how we were going to occupy ourselves (on weekends especially). Sometimes it was going to a museum (to see a child friendly exhibition or to do one of the weekend activities set up for kids), the zoo/aquarium, the park (went to various parks and playgrounds so neither of us felt bored) and would treat ourselves to a muffin/ice cream, etc at the end. Sometimes we went to the library, or swimming. Sometimes it was even something as 'sad' as getting a coffee (for me) and almond croissant (for dd) and going to a busy train station to watch the people rushing around/flooding out when trains arrived (the guards usually indulged us and let us past the barriers so we could sit on a bench).
I am not an especially crafty, or get down-on-the-floor-for-hours type Mum either, but I got a few books from the library that gave me ideas of games to play/easy things to make. Sometimes we'd make/decorate cookies (even if I secretly threw them away after). It is amazing the different things you can make with paper plates and a bit of coloured paper and glue, if you just have some ideas in mind.
What worked best for us was to get out of the house for at least a few hours every day. That way, even if we didn't interact meaningfully with anyone else, we'd had some fresh air and a change of scenery. The most important thing was to have a plan. Otherwise, it was too easy to feel lonely, isolated, listless, bored, sad, impatient with dd, days stretching out endlessly, etc.
As dd got a bit older (I think around 3.5), we began to have regular Sunday lunches with another single Mum and her dd. Those Sundays were wonderful because it gave us somewhere to go (or we knew someone was coming to us), there was social interaction for both dd and me, and it was a lovely and relaxed way to pass the time (especially when the weather wasn't good). Why not invite people over for Sunday lunch once or twice a month? Or could you invite a girlfriend over for supper during the week? You would have plenty of time to chat once dd is in bed (giving you some adult time), and wouldn't have to sort out (or pay for) a babysitter.
I think you have to acknowledge that what you want as a Mum is probably different than what you wanted as a single/professional person. And your relationships with old friends has changed dramatically (some friendships shift to a different way of being, and some fall by the wayside) as you are available to them in a different way.
I spent some time and energy staring at conventional families and wishing that had been me, and feeling sad for both me and dd that our lives hadn't worked out that way. There is real sadness and grief when your 'ideal' life situation has not worked out the way you hoped/dreamed. But you have to move past that, and find a way of enjoying the life you have. It might take a bit more imagination and inventiveness than simply being in a conventional family unit, but it can be done.
Finally, i doubt that your dd is feeling 'sad' now that you are back home from your visit with family. If anything, she is sensing that you feel sad and lonely, knows that you like having people around, that you feel isolated. Don't feel guilty. If you feel OK, your dd will sense that, and she will be OK.
So.......what is your plan for next weekend? Start thinking! And, good luck. (My reply is probably longer than your initial post , but hope you have found something useful in the rambling.....)