I keep my partner interested by being interested in them and attentive to them. I don't really feel like I have to go out of my way though - we have similar love languages, so it just comes naturally. Learning about your partner's love languages and attachment style can help. It sounds like you might have an Anxious attachment style, and what I've found is that LDRs tend to activate anxiously attached people a lot more than regular local relationships. I'd recommend checking out this book: Insecure in Love: How Anxious Attachment Can Make You Feel Jealous, Needy, and Worried and What You Can Do About It
I'd recommend checking out the book "Insecure in Love". It's for people with an Anxious-Preoccupied attachment style: https://www.amazon.com/Insecure-Love-Anxious-Attachment-Jealous-ebook/dp/B00KBEHMJ2/ref=sr_1_1?ie=UTF8&qid=1506325595&sr=8-1&keywords=insecure+in+love
Long winded answer coming but yes! I didn't even realize until my most recent relationship. I've dated a good amount, was always super insecure in the relationships, and usually broke up with them before they could break up with me (even though, realistically, they probably weren't going to).
Anyway, in my current relationship in the beginning, I was doing the same thing...constantly feeling like he didn't really like me or want to spend time with me, acting okay with it and holding that deep down, and then blowing up later. I knew I was acting "crazy" and wanted to stop it, but I just couldn't. Anyway, I googled it, came across Anxious Attachment, realized it fit me to a T, bought the book "Insecure in Love" and read it, and started practicing the strategies listed. Additionally, I explained to him why I acted the way I did, sent him this link as a suggestion, and would bring up my feelings as soon as they happened instead of pushing them down. He's been really great and patient and I feel like for the most part, I'm now in a "secure" relationship. Of course I still have days where I think he's going to break up with me out of the blue, or he's annoyed by me or whatever, but for the most part, I feel much more secure due to changes we both made.
Yes, I really relate to this - I think it has a lot to do with having an insecure attachment style, at least in my case. I found this book helpful: 'Insecure in Love' by Leslie Becker-Phelps.
This page has some useful techniques for dealing with stress/anxiety: Resources for Dealing with Trauma, Stress and Anxiety