You say you consider yourself to be plain, but most of us are at our age, without the help of make-up! It's unlikely that you aren't attractive, with two beautiful sisters, you're just comparing yourself to them, which is never good. And lots of people who aren't conventionally attractive are still considered beautiful.
Someone once said, "It's hard work being 'naturally' attractive", it's true! Build on what you have and do something different, colour your hair or have highlights, wear a little make-up, if you don't already, and sometimes wear clothes that aren't practical for cycling! Looking good makes you feel good and helps to boost your self confidence, which is attractive in itself.
I know it's hard to put yourself out there, but maybe message people on dating sites, even if you don't meet them, chat, flirt and be the sexual being you believe you aren't. It'd boost your self esteem and be a good distraction, and you'd also let hot crush know that you're available when you tell him all about it.
I know it's superficial, and possibly an unpopular opinion, but we're all peacocks, physical attraction is always the start. Catch his someone's eye.
You have another 40+ years on the planet, don't give up on yourself at just 50.