UML sequence diagrams can be particularly tricky to get right in a drawing tool, so specialist tools have sprung up for them. A couple of web-based tools specifically for sequence diagrams are
- https://sequencediagram.org/ This has a textual syntax for sequence diagrams alongside the UML diagrammatic syntax. It’s quite flexible and usable but you may need to look at the Help page to see how to use it – for example, it took me a while to see how to have participants in a sequence diagram such as “q:Question”. (Hint: participant … as…)
- https://www.websequencediagrams.com/