anapanasati: nimitta or ambient light?

Ambient light perceptions are impermanent and should simply be observed:

SN22.102:1.2: “Mendicants, when the perception of impermanence is developed and cultivated it eliminates all desire for sensual pleasures, for rebirth in the realm of luminous form, and for rebirth in a future life. It eliminates all ignorance and eradicates all conceit ‘I am’

Since ambient light perceptions are impermanent, they will cease:

iti72:2.3: Renunciation is the escape from sensual pleasures. Formlessness is the escape from form. Cessation is the escape from whatever is created, conditioned, and dependently originated.

Aversion to light is not the escape from ambient perceptions. Aversion to light is simply clinging to the dark. Both ambient light and the perception of ambient light are impermanent and will change and disappear on their own without intervention. If Mara whispers "meditate only in the dark", be very careful. Furthermore, consider that light is actually important.

Indeed, the Buddha encourages us to develop the radiant mind independent of ambient perception, open to all perception without clinging:

AN6.29:5.1: Furthermore, a mendicant focuses on the perception of light, concentrating on the perception of day regardless of whether it is night or day.
AN6.29:5.2: And so, with an open and unenveloped heart, they develop a mind that’s full of radiance.

Whether it is or isn't a vision of light i can't tell and i would advise you not to care about the classfication. Rather just discern it's arising, persistence, change & cessation as one would in course of general kayagatasati development.

I guess you are disturbed by this because you think that if it is ambient light then it is distracting you from the perception of breathing but seeing ambient light isn't really a problem lest you make it a problem.

Did you know that according to the Sutta the particular benefit of anapanasati is the cutting off distracting thoughts rather than seeing lights & visions?

He should develop perception of unattractiveness so as to abandon lust... good will so as to abandon ill will... mindfulness of in-&-out breathing so as to cut off distractive thinking... the perception of inconstancy so as to uproot the conceit, 'I am. An9.1

If your inclination & bent is seeing white lights, then there is really no point in maintaining mindfulness of the breathing over a perception of diffused light lest the perception of diffused light is causing distracting thoughts.

Also the in & out breaths are technically the internal air element and if you contemplate them as such and make it your inclination, then you can be expected to attain absorbtion based on the air kasina.

Whereas if you use the breathing merely to calm down whilst generally tuning your mind to the perception of light, to a pleasant feeling, to the deathless element or whatnot, then it can go either way once one is sufficiently secluded and it's not easy to guess what turns out to be the dominantly effective inclination.

For starters, be aware that anapana-sati (breath meditation remembrance) as defined in the suttas is quite a different practice.

As this is a forum about Buddhism in general, it would be helpful if you designate what tradition you're practicing "anapana". Most people I would think, would assume when you say 'anapana', you're referring to sutta based practice which is completely different. I've done the style of anapana you're describing before, and have experienced those same problems and much more.

My advice is, if you want to continue that type of anapana you're doing now, there are Taoist systems that do a much better job of it (working with visual light, third eye, states of formlessness and frozen stupor). If you're doing 'anapana' and having all kinds of problems that and have lost any connection with physical breath, how can it even rightly be called anapana?

(my notes with detailed studies on how sutta defines anapana, and how later Buddhism redefine into something else) https://lucid24.org/sted/16aps/index.html

As other answers hinted at, you having these worries and doubts about your experiences and what they mean, is a sure sign that you got too obsessed with the concept of progress.

This concept is just an idea in your head, a kind of hook you are on. Paradoxically, the only way to get to the next level is to drop this notion of levels and progress altogether.

Try and let go, and see what remains of your meditation.

We don't care about nimitta or ambient light. We care only the breaths.

The nimitta or ambient light is naturally appearing at focusing the breath. Thinking of light or nimitta is not thinking of breaths. Thinking out of breath is losing the meditation. Focus on the breaths. Don't think of nimitta or ambient light, although it is appearing so shiny.

When the meditation on breaths are in balance and powerful, the nimitta or ambient light will be stable and powerful at the breath together automatically without any attempt on them.

That strongest meditation level is when the practitioner letting go the breath and take the strongest & brightest light instead of the breath. This step is call "Vivicceva kamehi [letting go 5 strings, breath, etc., by getting the shine bright Nimitta instead.]" and "Alokasanna [Light Memory]".