My partner and I just opened our relationship and she's been with a few people since. At first our sex life with each other was pretty exciting but things have seemed to fizzle out recently. I'm naturally anxious that she doesn't find me so attractive anymore now that she's been with others but I also know that probably isn't a valid feeling. Any advice on navigating these feelings? It's not the lack of sex that's getting to me so much as the idea that I might not be attractive to her anymore.
You've done a good job identifying that these feelings are coming from your anxious tendencies, not empirical observations. Keep working on them from that perspective. Remind yourself that you are not psychic, and assumptions about the inner state of your partner's sexual attraction may not be accurate.
Identify what your partner does, or can do, to make you feel reassured that she is still attracted to you. Let her know that you might need some extra security around this issue and be clear about what she can do to help you feel better.
Read up about NRE in polyamorous relationships - this is a pretty common phenomenon. An established partnership's sex and romantic life often 'fizzles' into a dip when a new partner enters the picture. This is common, but can still be painful and disruptive. Being able to name and identify it often helps, though.
Sometimes, it takes a bit more intentionality and focus to re-ignite what has 'fizzled out' during NRE. Consider planning some date time for just the two of you or trying something new and sexy together (shop for new sex toys together, look into local sexy events, book a sexy photoshoot together, read erotica or watch porn together, take mojo upgrade together, etc.)
That anxious little voice in your head may try to convince you that it "doesn't count" if you have to ask, or if it's not totally spontaneous - but that's bogus. Long-term sexual and romantic relationships require cultivation and attention, and that's just part of opening your relationship in a healthy way.