Wa ‘Alaikum Salaam dear sister,
May Allah (swt) forgive you for your previous sins and continue to guide you on the straight path. Ma sha’ Allah, sister,  you are moving on the correct way by repenting and doing everything you can to get closer to Allah (swt) and by being grateful for the blessings that Allah (swt) has currently granted in your life with your PhD program.

 

There are many reasons why we are encouraged to marry in Islam, and one of those reasons is to provide that sense of companionship so that people do not feel this sense of loneliness you feel. Marriage completes half of our deen and, therefore, explains this sense of emptiness that a single person might feel.

 

If you are not in a position to get married right now, I would certainly encourage you to get involved in halal social circles that will provide you with some social support and help you to develop friendships with other like-minded sisters. Perhaps, there are some kind of sisters groups that run in your local masjid that you could get involved in?

 

Furthermore, according to researches, loneliness is one of the leading causes of depression, so feeling depressed the way you do is a perfectly normal response to this feeling of loneliness you have been experiencing.

 

You also say that you are completing your PhD abroad so you are away from your normal surroundings, and probably friends and family, too. This could also contribute to this sense of emptiness and loneliness that you experience. Thus, try to keep good contact with your family and friends back home to reduce the loneliness that comes from this aspect of your experience.

 
You should not ever feel undeserving of anything, otherwise you will not get the full sense of satisfaction and appreciation of your blessing. You have gone about repentance in the best way, the way we are told to. Remember that Allah (swt) is the All-Forgiving and will forgive all sins if the person repents. You should stay firm in this belief and continue to focus on the positives and blessings in you life.

 
May Allah (swt) accept your repentance and bring you the best in the blessings He (swt) has given you. May He (swt) continue to guide you on the straight path and bring you success and contentment in this life and the next.

Salam,