Here is my personal take on the situation. I too am 60+, 64 actually. I don't like to have more than one large project going at a time, however I don't count smaller things that I can finish in a few days.
A simple baby quilt, a small wall hanging, using up some scraps to make strip blocks; those are the kinds of things I allow myself to start anytime I feel like I need a break from the large project I am working on. Nothing with a tricky or complex pattern, maybe a panel to add a few blocks and borders around, or some simple 9Ps. Well, you get the idea.
This way, I stay true to having only one major project at a time, while still allowing myself some sides for variety. Sort of like having the main course of a meal, the roast, but adding potatoes and salad for balance and interest. No one wants to just eat roast all the time. :D

I don't worry about the small things anymore. I do what makes me happy. Although these kitchen curtains are a little trying, so then I go to my other projects. So have fun>>>>>>>>>>>>>>>>

I think what ever works for you. For the first four years I did quilting I worked on one thing at a time and I have learnt now to extend myself a little and I have different projects at different stages. I took some time to get used to doing it but I think in the long run it has helped me try more things and I still get them done - just over a longer time span. Whatever you do - it needs to be fun for you. There are enough things that "need" to be done and done a certain way. I am not going to restrict my hobbie and pleasure activity to please others. Enjoy your own style and tweak it if need be.

I finish one before I start another. UFO's would really stress me.

I try to keep one quilt going at a time. A least one Large quilt on my quilting frame, one baby quilt that I can get done reasonably fast and if I can't do that in front of the TV then I have a knitting/crochet project going. Thats what I'm doing now. The baby quilt is to sell and the crochet purse is sold also. Fun stuff! keeps me busy when I'm not working my income job as a nurse on the weekends. Oh, I almost forgot. I"m finishing valances for my great room. Just have to put on the gromet's.

I used to be a 'serial quilter' too but then something in my brain clicked and 'allowed' me to have multiple projects! You say you're worried about the quality of your work if you multi-task - I feel for me that I'm more likely to do second rate work if I MAKE myself finish something I've lost interest in than if I switch to something else. Everything will get done in the end - or not but hey, it's not the end of the world. :mrgreen:
The exception is commissions or gifts with deadlines or pieces for competitions. If there's a deadline, you just HAVE to get it done. The other stuff, well I do what I fancy and when because even though I now am partly doing this as 'work' at heart, it's still a hobby so if I'm not loving it, what's the point? :-D

And I can add to that, 2 or 3 different knitting items :)

I actually try to have three projects at the same time. One in what I call each stage of the process. For example, I have one that I am cutting out, one that I am in the process of piecing and one that I am in the process of quilting. That way I don't have to sit and just do one part of the process until all done. If my fingers hurt from quilting, I take a break and do a little piecing or cutting on the next two in the que.

Are we twins?

I didn't start quilting until about 15 years ago. Working for Hancocks part time gave me opportunity to be in a fabric store more than I should have. After 11 years of working a full time job as a secretary and part time at the fabric store I have more than enough stash. I love scrap quilts the best I have about 5 or 6 projects in some level of progress. I guess the reason I love quilting is that it satisfies my creative nature. Unless I'm really "hook" on a pattern or have a deadline I can go from one project to another without much problem. I do get project finished and seldom in a hurry to get it done. Like I said unless there is a deadline. Quilting is the one area of my life that I have control over, so I'm seldom in a hurry. My son is 35 and I've been divorced since he was 3. Quilting has been my drug of choice. It's a very creative outlet for me. It's helped me break many bad habits. Such as I have to be perfect or no one will love me. I use to think I had to tell were all my mistakes were or I'd be cheating. Well if i don't tell you you probably won't ever now. And most importantly if you a true friend, you'll still love me any way. When I first started quilting I thought I should be able to relax. I wasn't relaxing. I've since learned to relax and discovered that I'm really very good at color selection and very creative. So my advise to you is, don't forget to breath. It's very important. Mix up the rules. Stay with the project until it's done or don't. It's you hobbie, it's your outlet, ENJOY!!!!!