。
I miss my mother so much I can't stand it.
Literal
Mother [with-に] want-to-meet [so-much-that-can't-stand-てたまらない].
~てたまらない attaches to the te-form of a ~たい-form adjective to express an unbearable, overwhelming desire or feeling. Here the chain is 会いたい → 会いたくて → 会いたくてたまらない ('want to meet so badly I can't stand it'). たまる means 'hold up / endure,' so たまらない is 'can't endure it.' 会う takes に for its object when it means meeting a person. The speaker ('I') is entirely omitted — emotionally expressive Japanese often drops even 私.