I have really bad anxiety and depression and whenever my partner is away or not responding I start getting really negative thoughts and thinking they’re with someone else and ignoring me because they think this other person is better than me and it’s really messing with me and my relationship. What can I do to make this go away?

Ooh! I know this one! The answer is therapy. Specifically DBT and/or CBT designed to address anxiety, obsessive thoughts, and intrusive thoughts. This is a well documented phenomenon and we have lots of tools to help address it! Talk to a professional for help, and in the meantime, you can also find self-help workbooks and apps for this. Check my resources here!