Why do I have this tension in my chest thinking about meeting my (ex) girlfriend?

We went on a break a few days ago because I was confused and really feeling mixed emotions because she kept talking about marriage and other things I wasn't sure about since i'm only 18.
Since then we've both found it really hard and I've missed her an awful amount, so we're speaking again today and talking about meeting again and seeing how things are. I know she wants to get back together and I really do feel like I want nothing more than that too- I have the warm feeling I used to whenever I speak to her on the phone and keep smiling at the thought of hanging out with her. I've literally not stopped thinking about her at all.

However I'm also really scared because I want things to work out and I don't know if i will have similar doubts. I don't think I will because we've spoken about the key things that were making me doubt the relationship anyway and have agreed to sort them out. (e.g. we literally never went out anywhere and i felt like we were losing the "spark" which was probably because we spent 90% of our time together inside doing nothing)
I keep getting this feeling of tension in my chest which feels similar to the tension I was feeling when I was thinking of breaking up, and it is also similar to the feeling i've had before when she's been busy for a long period and not answered my texts and stuff where i've been worried.

I think when we broke up and I was instantly scared i'd made a mistake and a rushed decision I felt the same thing and now i'm feeling it again i'm terrified to what it is.
Does it just mean i'm worried about what is gonna happen when we meet?

It's just like a dull ache in my chest.
I can't get her out of my head and just want to hold her again and every time we speak and i hear her voice and laugh again it slowly goes away. Do you think it is just stress/anxiety or is it a sign that things between me and her won't work out because my brain is saying they will because I feel like i love her a lot :frown: