Wow, this looks like this is really hard for you.
As for my opinion on how you can "love life," you have to understand that there is also a balance in between. There are three different types of ways to feel/look at life: some people have really hard lives, and some people seem to sail through, but there is also the type of person (and I think most people are like this) who has good days and bad days.
I can see that you are going through some intense challenges, and that is probably tipping the scale on the negative side of the spectrum. However, in a practical sense, if you want to be happier with your life, you can't keep looking at how everyone else (who you believe) "loves life."
You mentioned that people always use the words "she loves life!" Or, "I love life!" Some of these people may really love life, but most people do not spend every day or even most days thinking about how great their life is. Everyone has challenges, but people like to show a happy face to the world. It's not good for shidduchim otherwise... It is likely that these girls are just in a great mood, do have an easy life, or are just being bubbly.
You should try to remember that and not feel so bad - like you said "it hits you with a pang."
It seems to me that you spend a lot of intellectual energy and time on this subject so it must bother you a lot.
Maybe if you tried not to focus so much on how you wish you were at that level of "loving life," and just lived each day and focus on the minute details of good things that happen, it would be easier to be happy, instead of focusing on the broad picture/feeling that you think is the ideal.
The ideal of life is to grow and learn from your challenges. Look at our Avos and Eimahos - they certainly did not have easy lives, and I doubt that they would say that they "loved life," but look who they are! (*Yaakov cried for years after Yosef was sold...) Look what they became! Maaseh Avos siman libanim... We go through challenges too, and our job is to focus on growing from them and staying strong.
You said that you know "loving every minute of life is essential." I don't think that is true though. How can anyone?
You don't have to love life, and I know that sounds crazy, but just focus on how strong you have become and are continuing to become from all of your challenges! Even the ones that seem insurmountable.
(*That's not to say that I think it's okay for someone to be depressed or unhappy most of the time... Like I said, try to find a balance...)
There is a quote, "Everything is good in the end. If it's not good, it's not the end yet." (I think by Rebbetzin Jungreis) Isn't that powerful?!
The girls who "love life" don't have the zechuyos and strength that you gained because of your challenges!!
And just remember, no one always loves life, however much they appear to. It is a good ideal, but not one that should make you despair if you feel that you have not attained it.
Hatzlacha! I hope things become easier for you, and that you find it easy to grow and love life!