Hello Everybody....I´ve been following this site for a very long time but only now I have decided to make my first post.
First I will tell you a bit about myself and my acne. (Hope its not too long to read!! 
)
So Im a 23 year old girl from Portugal who has suffered from acne ever since I was about 9...they started a small bumps ,and then when I was about 13 they really got horrific...on my chest,back, arms, scalp and my face was covered with deep painful inflamattory acne. I was diagnosed with PCOS and put on Diane35....my acne slowly got a whole lot better, the PCOS cured,my back and chest slowly cleared up and also my face over the years, and I got from a self conscious depressed outsider to beautiful girl with lots of confidence.
Well this part is something Im not proud of but feel I have to talk about in order to tell you the whole story...
When I was 18 I was with this guy and , over using antibiotics to treat an infection I had at the time, the pill got its effect useless and I fell pregnant. I aborted and my acne came back, but just on my face. I got back on Diane35, went my dermatologist who precribed
Accutane 20 mg for 6 months.This was in January, and well,in Portugal , by April, the sun is very strong, and I was taking my drivers licence back then , and soon got all burned from Accutane. I, stupidly, thought my skin was already fine (because I had no acne anymore by month 3) and stopped the treatment. Skin kept beautiful
Shortly after , there was this thing on the news where girls in France had died from Diane35, all my age, and pill had been taken from the market. I got really scared and stopped taking it. Skin kept beautiful for about 8 months
Than I started getting some pimples on my lower cheeks, and for some reason, a lot on my buttocks , which I had never had.
I got a new boyfriend (which I stil have at the time)
Well, they still disturbed me and I, stupidly again, went to the pharmacy and asked for a pill that doesnt kill me and that its good for acne. 3 weeks into Valette and my face was filled with the most horrific terrible cystic acne I had ever had
I spent that Summer laying in my boyfriends arms and crying. That was my first big depression.I stopped Vallette and after about 4 months my sking got back to its usual self.
I got out of my depression
Then the shit hit the fan.
I got pregnant again
I aborted
I got 1 period
I got pregnant again
We did used condoms, they just broke a whole lot 
I know it sounds mindless but we did try to be as careful as we could, even started wearing 2 condoms at a time, but we were both unemployed, and good condoms were expensive and we used the free ones from the health center
Moving on with that,,,,,,
Ever since Im plagged with horrific cystic acne all over my face, back, arms, chest and butt. *Mod edit*
Its now the 3rd summer of my 20`s dealing with this crap
This is what worked for me:
Doxycicline, cleared me up beautiful or 8 month then I grew Immune
Mynocicline,cleared me up beautiful for about 5 month then I grew Immune
Im also taking Cyproterone Acetate alone, 100 mgs for a month and a half
I have no period since January
Ive tried Erytromycin,Clarytromycin,Bactrim, with no sucess
I ordere oxytetracycline with precription from [removed], that comes from [removed]
It i clearing up my skin
I now placed my order from Tetralysal (Lymecicline from [removed]) again
(Because they dont sell here in Portugal)
Im so desperate, unhappy, fearful *Mod edit*
Any avice and thought with be apreciated
Thanks a bunch
Maura 
These pregnancies and abortions are so bad for your body and is liely what is sending your body into an upheaval. You need to find a safer and more reliable method of contraception or abstain from sex. Using more than one condom is also not a good idea. It can take months to years for your body to get back to normal so i would recommend seeing a derm or at least you OBGYN to discuss your options.
I am not a doctor but it sounds to me like your acne is completely hormonal. I say this because the times you mentioned that your skin got worse, when you had an abortion or stopped taking birth control pills, are all times when your hormones are thrown out of wack, plus the fact that when you took Diane 35 it cleared you up.
No antibiotics will clear it up and keep it clear, plus it is not good to be on antibiotics for a great length of time (again I am not a doctor but everything I've read suggests this).
In my opinion you should find a birth control pill that works for you and will also clear up your skin, Valette was obviously not the right one, you may have to try a few as they work differently for everyone. Apart from helping your acne you need to be on birth control for the purposes of birth control as it does not seem wise to keep having abortions (again, just my opinion).
Or alternately look at natural ways to balance your hormones through supplements, vitamins etc.
But please address your birth control issue as well.
Good luck.
Is there tetracycline cross resistance real?
Yes - becoming resistant to one tetracycline makes it a more likely that the bacteria causing your acne will become resistant to other tetracyclins (like doxy, mino, etc. as well). I agree with the above posters, you may need to get on a hormonal regimen, I am not sure what is available in France, but Drospirenone/Ethinyl Estradiol daily pills work well for many people with acne. You may also add spironolactone, which is indicated in the treatment of acne for women with PCOS. Also, using BP with oral antibiotics can significantly reduce chance of resistance.
I don't think you need to beat yourself up about the decisions you've made regarding your own body - but you need to use condoms correctly to protect yourself against STDs as well as unwanted pregnancies. Always use latex or polyurethane condoms of the correct size. Check the expiration date on the package, they are usually only good for about one year. Make sure you are storing them correctly, keep them away from heat and moisture - for example, a wallet or a glove compartment is a horrible place to store them. DO NOT USE TWO AT THE SAME TIME! This actually increases the risk of malfunction by a whole lot. Make sure you are are using lots of water-based lubricants (oil-based lubricants should NEVER be used, they weaken condoms and increase the chance of breakage). If using spermacide, make sure it is water based as well. Of course, do not re-use condoms, and be sure to use them the entire time you are having sex.