There are a few factors that can lead to a feeling of tightness in the throat while feminizing the voice. The most common issue I see is people who are putting in too much activation energy - or, to put it simply, trying too hard. The voice is one of these tricky things where the point of diminishing returns can sneak up really quickly and once you pass it, the more effort you put in, the more tension you can create around your instrument.

A question I like to ask my students who are trying really hard to achieve the result is "how would you do this exercise if it were easy"?

The mind is a very powerful thing, and the stories we tell ourselves have a huge impact on our behaviour. If you approach your exercise with the idea in your mind that it is really hard to do, then you will put in an amount of effort that matches your perceived difficulty level.

Try this: breathe for a few seconds before your exercise and ask yourself "How would I approach this exercise if I were sure that it was easy to do?" Did you get a different result?

I hope this helps!