I think my boyfriend is losing interest?

Oliviaaa3
We've been together 6 months and recently I've noticed a change in him. For the first few months of our relationship he was so sweet, always wanted to talk to me, see me, call me etc. As time went on, he began doing it less.
His ex girlfriend and me and friends and she told me that the same thing happened in her relationship until the point where they had to break up and my boyfriend apparently loves in hindsight and doesn't realise what he has until it's gone (that's what his ex said as he told her that's why they broke up).
I've noticed that he's been talking to this other girl, she's 3 years younger than me and him (she's 15 and we are 18, they know each other as we are in the last year of school and she goes to our school). The girl has a boyfriend but I notice my boyfriend and her talk and they have done for about a year. I can't help but feel jealous and upset, and I feel like sometimes he would prefer to talk to her over me. She knows that we are dating, and I've spoke to her a few times and she's really nice, I just have this horrible feeling my boyfriend may have feelings for her as they talk a lot (I've seen the messages and they only joke around, they don't flirt).
I have this horrible feeling that he's losing interest in me and I've tried telling him multiple times about it and he tries to reassure me he's not and that he loves me etc but his actions prove otherwise. I feel like the more he pulls away the more I cling onto him. I get upset whenever he doesn't message me, or ignores my messages and then I worry in case he's talking to that girl. I feel as if my life revolves around him and he's always busy and I never am. I feel like I drop everything for him but he'd never do the same back. The girl he's talking to (yes I know the age difference is weird and I don't understand why they talk), she's very laid back, funny etc whereas in the opposite and it makes me jealous in case he prefers her.
What do I do?
I think my boyfriend is losing interest?
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