Women and weight gain after 40!

I decide to post this topic because I have had several conversation lately and the outcome has been frustrating for me. Let me explain - I am 47 years old going through the menapause years. It has been a struggle to maintain my healthy weight that I have had all my life. ( Which is a 5 pound range between 123 and 127 - I am 5ft 6in) I have always eaten healthy and maintained an active life style. Once I hit forty and also had some personal problem my weight went up to about 150. I was able to stop the gaining and increased my exercise and lowered my calorie intake and was able to get down to 125 but not able to maintain it. I can maintain 130 to 133 fairly well but with a lot of effort. I exercise - strenght training and cardio at least 4 times a week but usually 6 times.( I usually exercise at least an hour a day) I also just remain highly active. I watch my fats and carbs and eat mainly clean. I vary my calorie intake between 1300 to 1800 calories a day, depending on my activety level that day. I think that is alot work to maintain my weight- and I have been accused of being obsessive about it. So this is my frustration. I know other women who have gained the weight after 40 and say that is just the way it is. That I am not going to be the same weight that I was in my 20's. Or I get the muscel ways more than fat, it is water weight, it's hormones etc. That being healthy is not a number on the scale etc. It is not that I may or may not agree with the statements - I want an answer to the statements. Why??? Why do I have to gain weigh after 40 - My point is - I realize that my metbolism may slow up as I get older - ok fine - I exercise alot more than I was 20 and I decrease my calorie intake - so shouldn't that counter act it. I am not opposed to working for it. Also I am not opposed to settling for weighing 10 pounds more than in my 20's but I would just like a REAL answer for it, without everyone thinking that i am obsessive or that I have some kind of eating disorder or something. I have posted on a maintainance thread about this vary thing and I realize that other women have this problem too. But I was hopeing to get other opinions and may some answers. So can women weigh the same in their forties as they were in their twenties if they are willing to work for it.? Or is that just an impossible dream that will just end in frustation?