I think you are around the national average. Maybe 10 pins or so low, but that's only one spare per game.
Most women bowlers I've seen in mixed leagues are there because their boyfriend or husband bowls. They are just looking for something to do with their significant other. If you are looking for something more then that, then there's always coaching. But no matter how you look at it, increasing your skill takes more effort then presently put forth. If you cannot do that, for whatever reason/s, then enjoy where you are and do the best you can. Its a tremendous hardship for women to gain skill at anything once you are an adult, married, with job, with children, husband etc....... Doesn't matter if its bowling, softball, violin lessons. Your time is usually pretty scrounched. Making room for more time/practice in any hobby/sport is difficult to nearly impossible for most adult women.
I'm not saying that men don't have time constraints as well. Only that women view the improvement levels differently then men. I think more men can bowl, let off some steam and be happy at averaging 150, drinking a few beers or sodas with no thoughts of getting much better, then women can. I think most women, when they put their minds to getting better, find more road blocks from their lifestyles. Men seem to accept their time limitations better then women. Women tend to overdue and burn their candles at both ends.
Bottom line from my perspective of your post is that you wish you were higher average. But not really sure how to get there. Its going to take some coaching, and practice, time on the lanes in games (say 9-12= a week) where you just work on improving skills your coach as guided you on. Aimlessly practicing is not productive.
Another jist that I got from your post is that if 140 is normal then you are good to go. What if I suggested to you that 160 was normal? Now what? :-)
Time to get going GIRL!!
Erin