I spent several years in your exact situation, albeit with more than one partner to whom I was "secondary". My advice to you (and what I in fact did) to cope with needs unmet is this: evaluate each relationship on its own merit, not on what you hope it might become under future circumstances. Decide if you are happy with it NOW under its current limitations and benefits. This is really what poly is all about.
If it works for you now, then great! It's a relationship that's working.. no need to fix or leave it. If it's not working, then figure out why not and maybe you can address changes with your partner. If the reason it's not working is because you only want a primary relationship, then poly (or at least being poly as a secondary partner) is very likely not for you.
One final word: become your own best friend, your own primary partner, and love yourself as much as you would a partner. This can help a lot with figuring out and fulfilling needs. Many needs we have are actually best fulfilled by ourselves and not a partner (surprise!) so the gift of time alone is actually very precious.
Good luck, and remember that not even a primary relationship can protect you from feeling lonely if you're not already secure about sometimes being alone.