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This question doesn't sound stupid!

Sometimes BPD is diagnosed in younger teens, but you're right that for the most part it isn't diagnosed until the late teens or adulthood.

I of course can't diagnose you with BPD and I don't think self-diagnosis would necessarily be helpful, but there are definitely some signs that you meet. The Australian BPD Foundation Limited describes the DSM-5 diagnostic criteria of BPD here.

You don't need to have had a romantic partner to be diagnosed with BPD. I'm diagnosed, and although I have problems with my own diagnosis because I don't think it fits, I've never had a romantic partner. When it talks about relationships, it's talking about all forms of relationships, not just the romantic ones. So friends, family, peers, etc. Basically you go through periods of idolizing the person and thinking they are the greatest, and then the way you feel about the person shifts and, as it was described in in the link above, you devalue the person and think more negatively towards them.

A lot of the time (although not always) there is a trauma component around BPD, meaning that you have had trauma(s) happen when you were young that may contribute to how your personality was formed.

Honestly, with the self harm, your reasons are valid too. Some people have a harder time handling emotions, even the positive ones. It's important to find other ways to express what you are feeling other than self harm. One thing that I've been told to do a lot is to hold an ice pack or even take a cold shower, kind of as a "reset" to your body and its functioning. You can do art, writing, or other creative activities. Intense exercise can burn off some of that energy and stress. There's more alternatives here.

To make a long story short, yes, I think you should bring this up with your therapist. Even if they don't think it's BPD, they can tell you what they DO think and go from there. If they do think it's BPD, they can also explain why they think that as well. Even if they don't think it's BPD, opening up about the symptoms and things you are experiencing can be really important because it will help guide your therapist on how they can best help you and meet your needs. Let them know about the dissociation, the lack of clear image about yourself, the self harm, etc. It's important that they know so they can help you.

I'd make an outline of what to say to your therapist. You can write it down somewhere and either show your therapist the outline, or use it as a guide for what you want to say to your therapist. Some main points to consider are telling your therapist that you are worried that you may have a mental health disorder beyond depression or anxiety and why. Tell your therapist that you are experiencing the symptoms that you mentioned above. Don't sugarcoat how you are feeling and don't downplay any symptoms you have. Tell your therapist that you were doing research into various conditions and you feel as if you could most closely relate to BPD, but that you are open to other possibilities as well, and that you would like to learn more about what is happening and gain a better understanding and more insight into what is going on. If you don't feel comfortable verbally telling her, you have the option of writing it all out in a letter similar to how you did with us and then handing it to her or emailing it to her. This would ensure that you are able to get everything out without nerves getting in the way and without forgetting to bring up a point.

I hope this helps at least a little. Best of luck on your journey.

Dez