Staying positive vs frustrated

How do you stay positive when it feels like progress on the bike is so slow?? I got a gravel bike earlier this year (after not riding my road bike for years because I’m not a roadie lol) and got a MTB last year. I definitely notice my progression on my MTB vs last year but despite putting more miles and time in on my gravel bike, it’s much harder to see my progression. I occasionally ride with a gravel club which is mostly dudes and a couple VERY strong women all of whom are faster than me (I’m averaging 12-13mph on gravel rides and they’re at 15-16mph) I’m constantly trying to catch up. My boyfriend rides with me and is also faster, but sticks with me on the group rides, which makes me feel bad for holding him back even though he says he’s totally cool with it. Maybe I should ride by myself more until I get stronger and faster?? I’m also a little worried that I won’t get stronger or faster- I’m in my 40s (as much as I don’t want to admit it). Is there some threshold where you can’t get stronger and faster?? Ughh…. feeling frustrated 😕