高校時代からつきあっている彼氏はいるものの、つきあいが惰性になっていると感じ、不満をつのらせている。

、、。

I've got a boyfriend I've been with since high school, but I feel like the relationship has become routine, and my dissatisfaction is growing.

Literal

High-school era [from-から] dating boyfriend [topic-は] exist-[even-though-ものの], dating [subject-が] inertia [into-に] becoming [quotative-と] feel, dissatisfaction [obj-を] am-building-up.

ものの is a somewhat literary contrastive conjunction: 'although, even though X, Y' — similar to のに but more distanced and contemplative. つきあい here means both 'dating relationship' and 'ongoing association.' 惰性 is the noun 'inertia, force of habit, going through the motions.' つのらせる ('to build up, to intensify') is the causative of つのる ('to grow stronger'), a formal/literary verb. The whole sentence is the voice of a narrator reflecting on a stale relationship.