Too attached to my boyfriend? Help

(I have not been professionally diagnosed with BPD, but I'm 99% sure I have it just FYI)
My boyfriend and I have been seeing each other for around 8 months. I love him very much and I know he cares for me, but whenever I say 'I love you' to him and he doesn't say it back (he says it's too early??) I get very sad.

He doesn't want to have sex as often as I do (every other day I guess) which leads me to thinking that I'm too ugly for him. And when he does want it, he makes me lay on my stomach because it 'feels the best' to him but I can't help but think that it's because he doesn't want to see my face and that upsets me too.
Basically I don't believe that he's sexually attracted to me and that makes me extremely nervous, and self conscious. I put on makeup every night before sleeping so he wil not see me without it.

I see him every day and cry when he doesn't sleep over because I think he hates me. (He doesn't know that lol)

Yesterday he asked me if I wanted to do a threesome w another girl and I said no. But that means he wants to have one and I've been freaking the hell out over that and wanting to cut myself because I'm not good enough for him.

I constantly buy him things, like food (sushi. So expensive ugh), video games and even an xbox once, and a 200$ ring (that I ended up losing somehow woops) as a trust ring so he knew he could trust me (long story) anyways I buy him things so he won't leave me, even though I'm very stingy with my money (VERY) and broke.

Help!! I love him so much but maybe too much. I cry everyday because I'm so scared I'm not good enough for him or I'm scared he doesn't like me or blahblahblah. How do I stop being like this?

By loving yourself. What would life be if he went away, try to write the situation. My dbt manual has a series of self help chants which say

Be fair to your feelings
Dont apologize for being alive
Just because i say no doesnt mean i'm a bad person
Just because you're a friend doesn't mean I have to agree to your every request
Be fair to your own feelings and don't act helpless and cry when you're not helpless
You have the right to stand up for small things just as others have the right to stand up for big things

Also whenever you get anxious and miss him, read all those messages you sent earlier. If he wanted to leave you, he would've left by now.

remember that even he has a life and a right to love you or leave you. write down the worst things that could happen if he leaves you.

Start thinking about yourself. There is no virtue in sacrificing yourself for the sake of others all the time.

In my opinion you should communicate with him to clarify what each of you wants from the relationship. If you want a monogamous relationship and he wants something else, there may be a fundamental incompatibility in your relationship. I think you might also want to stop buying him expensive things. Does he ask you to buy any of those things, or do you do it out of desperation when you find out he wants something?