Dear Syed,
I can imagine what you are going through right now. Please know, as difficult as it may be for you to understand this right now, taking care of yourself is your priority right now. Think of all that you and your family have been through to get you where you are today - you have fought a life long battle with thalassemia - and you must continue this fight regardless of your relationship status with your girlfriend.
Many thalassemia major patients are parents of healthy children. It is possible for you to have healthy children. Depending on how well chelated you have been, you're future partner should be able to achieve pregnancy - if you have any difficulty - you may be able to achieve pregnancy through assisted reproduction. I know of young men, who have considerable iron overload - yet they are still able to have children. Keeping yourself healthy and maintaining low iron levels is a responsibility you have to yourself, your partner and your future children - because they will need you.
Thalassemia should not be a reason for someone to reject you as a marriage partner - these days - it is very possible to live a normal and healthy life even with thalassemia. We provide a lot of guidance in terms of medication, transfusion regimen and supplementation on this website that can help you achieve a long and healthy life.
As for thalassemia status of your children - if your girlfriend does not have any form of thalassemia - then your future children will be carriers of thalassemia - but should be able to live a very healthy and normal life - and there is no risk of them having thal major. If she is a carrier of thalassemia then there is a 75% chance of them being thal major. There are methods you can use to prevent and detect thal major in any case.
Most importantly, - you have thal major - you are NOT thal major - therefore having your children be like you is not a bad thing. Her parents should feel very fortunate that someone loves their daughter as much as you do - and that you are gathering all of this information so that she can live a fulfilling life.
Welcome to thal pal - and let us know if we can help you in any other way,
Best,
Sharmin