Boyfriends best friend

I don't like my boyfriend's best friend, at all. I tried to at first, we were okay and talked quite a bit. I'll try to shorten this as much as I can. I know she wouldn't try to do anything to harm our relationship on purpose, I'm just tired of bad things happening. I never seem to get a break.

My boyfriend's best friend is his ex girlfriend, their relationship was short and extremely toxic. I found out they were together a while ago, he kept it from me because he was scared. I would have been fine if he told me sooner but we've been together for 2 years. 1 year long distance, then I come here a find that out.

How'd I find out? Well I was out with her grocery shopping, we were talking about him and I was expressing how he makes me feel like shit sometimes. Then boom she says something like "yeah I broke up with him because he was lazy and we didn't get along." Then for the rest of the night out I was quiet, kept to myself. While she's completely oblivious of the issue.

Once I got home I asked him why he didn't tell me, he had a breakdown, he said he'd block her out of his life. I said no because I didn't want to ruin their friendship. He said they swore to forget about their relationship cause it was so bad. I was already depressed due to moving in with him and leaving my family then that, I was horrible for weeks. Months go by then another argument because of her, she brings up some if the things I vented about. Told him I said "well she said she feels like she pays for everything" which is not what I said to her. I said my boyfriend makes me feel like I pay for nothing.

Another argument happens, he walks in the house all angry and telling me we need to talk. Asking why I'm talking to her, that's his friend not mine, and if I have a problem come to him. I do speak with him about issues, he usually doesn't take it seriously or gets angry so I stay quiet. He said that out of anger and eventually apologize when I ran off crying. Another argument started by her. She's just, I hate to say this, not the brightest. Doesn't understand what she does wrong. My boyfriend is always trying to push me to be good friends with her and I tell him all the time that she's his friend not mine and it's okay. I just vented to her once because i was in a tough spot. Left my family, constantly being yelled at, I have no friends, so i thought maybe it'd be okay. I'm not jealous just tired.

Am I wrong for this? Don't tell me to break up with him or say he's cheating, im not going to and he's not. Sorry this is a bit messy, my phone is acting up and types random shit for me sometimes. Takes like a full second to type a letter.

Someone asked "why ask" on this post, I ask because I'm venting and don't know what to do sometimes. I'm way younger than my boyfriend, he has much more experience than me. I just want to ask some women about what they would do in this situation. I feel alone, I can't really talk to my mom. I don't have friends or a sister.