I’m in crisis and is needing help

So I have had a borderline personality disorder diagnosis since I was in year seven (now in year 12) and I know it hasn't really affected me much as I have gone through high school. I have noticed with the help of a therapist here and there when it becomes more noticeable that I am "acting up". Recently it has gotten very bad since I broke up with my ex in January ( I know that we are now in June). They had been my best friend for three years before that and we had dated for two months before they dumped me. I have not been able to let go of them and they have blocked me on everything even though I have not tried to ever make contact with them. I am doing what my therapist is saying and is writing letters to them whenever I feel at my worst and then burning them but this isn't helping. I need that contact and connection to go forward. I can't see me living my life without them. I should also mention that I have been dating someone else for four months now and we are still together but there are little things that are making me feel like they are going to leave me, with leaving me on read and just not wanting to keep in contact at least once every day just to make sure that they are coping. I feel like I am cheating on her because I am so caught up in my feelings about my ex and all I want from them is to have their friendship back. We were so close those three years and knew everything about each other. I really don't know what to do. I am so torn and am feeling worn out having this taking up my thoughts 24 hours a day. I love my current partner with everything and am wanting this relationship to go further but I can't continue life without my friend.